Yes, but do you realize how much variation in playstyle and play goals there still is/was between groups given those parameters? I mean, correct me if I am wrong, but a run-of-the-mill thief would not be considered viable under your parameters.
Do you actually think most of the rest of us who played found that to be true? If not, we ought to consider why others found it viable, and you did not.
I played a thief pretty much at every opportunity. And no, they aren't viable. This wasn't something that was immediately obvious to me at first, and I certainly had lots of enjoyment playing a thief. When you first start playing, especially as a kid, this is all so new and wonderful that literally anything we did was fun, including monotonous hack and slash.
But the longer I played, and the more I thought about it, the more I realized that I was playing a character that couldn't pull their weight. Again, this wasn't an immediate realization, it came about from having a lot of experience with the class both as a player and as a DM keeping track of things. Most of the time I played I was focused on my ability as a player and I just sort of had always assumed that also there was this character class that enabled my favorite play style. But my first big revelation was as a DM after I introduced a house rule that XP other than XP per hit point/damage inflicted was shared. (There were a few other ways I'd been allowing individual XP as well, RP awards, story awards, individually earned or stolen treasure.) It turned out when I started tracking it that the high strength fighter was doing more damage than the other 5 characters in the party combined, and after that revelation I noticed the thief in the group honestly didn't have much to do despite my attempts to provide utility. Whether the player was aware of this I couldn't say, but it changed the way I looked at the game.
And as a player I started to notice a few things, especially as we got to higher level.
a) Even in a round where I got a backstab in, I typically did less damage than the party fighter did every round. If I could backstab every round, I'd still be a bad fighter. I wasn't really helping in combat. In fact, the party would be better served by a reasonably skilled 5th level fighter henchmen than my 10th level thief. On average a fighter half my level was better in me in every single metric - hit points, THAC0, damage, and relatively quickly most saving throws.
b) All my saving throws at low level had been pretty darn good, but the class actually functioned more or less the opposite of a M-U in that as you leveled up you got relatively worse. You had a linear power increase that was much much slower than fighter classed characters. I had always just assumed without questioning it that this slow rate of advancement was made up for by the fact that I could level up more quickly, resulting in a balance. But then I noticed that at most I was keeping about a level ahead of even the M-U in the party, and my assumption of faster advancement was blown. Mathematically I needed to keep at least 2-3 levels ahead of other classes just to keep up with them, but that really didn't happen. For most played levels I was 0-1 level ahead of the party. And it also became obvious to me as a DM as I was doing encounter design that the thief had no good save. I had been looking for something to serve as what we'd now call a reflex save, but there wasn't an obvious candidate for that in the list. Some other class always had a better save than you. The best I could do was call for roll under Dex checks, but then no one really got better at those.
c) It had always been obvious to me that the success of the class was primarily based on the players wit. That was one of the reasons that I liked it (aside from I think normal youthful rebellion). At low levels your 'thief skills' were so unreliable and the consequences of failure so high, that the correct move was to almost never use them. As has been pointed out humorously in popular media, it would have been smarter to have the party fighter blunder into the trap and spend a spell to heal him, than risk a disarm traps roll as a thief with basically no hit points. So disarming traps was something I usually did without regard to the skill. But if I wasn't actually going to use the character's skills, couldn't I in fact do this job as a fighter? At higher levels, your thief skills became reliable enough that you could start to rely on them as a saving throw, at which point you could be a better thief than the party fighter. However, by this point your thief abilities had been completely and in all ways outclassed by the even more reliable abilities of spell-casters. For example, while I could climb a wall with some reliability, a M-U could cast spider climb to climb walls with utmost reliability and perform feats of climbing I could never really aspire to (like hanging upside down from the ceiling). Or a M-U could cast fly and climb without a wall. I could somewhat reliably 'hide in shadows'. But an M-U could cast invisibility and hide with utmost reliability, including while moving and without concealment. I could find traps with some level of reliability, but the cleric could cast Find Traps and find them with greater reliability. I could hear noise, but both the M-U and the cleric had a variety of divination spells that outclassed anything I could do. I could open locks, but the M-U could knock open doors reliably - including in situations where I had no real chance of success. I could disarm traps, but a M-U with an unseen servant had vastly better scope to disarm traps safely using their wit and better dungeon hygiene than I could ever manage. I could use wands, but by this level an M-U could in theory actually make a wand. For a while I justified to myself that I still had the valuable role in the party of conserving the spell-casters very limited spells slots. But none of the spells I listed was in fact a high level spell. They were all relatively minor spell slots, and after a while I started to realize that even if I could in fact conserve a spell-casters spell slots with good play, on an average day the number of spell slots I actually conserved were less than I would have were I an equivalent level spell-caster. If I was a M-U that specialized in the sort of magic I was trying to conserve, I'd not only do my job better, but be able to say throw out a fireball occasionally. I was in fact a bad spellcaster as well as a bad fighter.
At that point I started to realize why my fun playing the class was diminishing, and my deeper understanding of how the game actually worked only increased the frustration further. My most beloved character was a multi-classed Thief/M-U, and that character was able to do what the thief class couldn't do on its own - provide me tools for my creativity that let me actually do far more creative things than I could ever do with just my wit. It also meant that I was no longer wholly reliant on getting the DM to rule favorably on my actions, because spells were packetized narrative force that let you essentially state thing about the fiction. (Of course, at the time I didn't have any of that language to describe why this was better.) So my agency and ability to succeed were increased by playing the more viable character.
And I certainly didn't need to play a non-viable character to prove I could succeed anyway and so self-validate in that manner. I'd had measures of success before I had system mastery/understanding.
This has nothing to do with the assumptions being unreasonable. It has to do more with what your definition of "viable" really is. What are the expectations that mark the difference between viable and not?
Well, I'm not sure I can put them into words succinctly, but I think I just did a very good job of explaining why the thief in particular wasn't viable. The close I can get to answering you is that for whatever your goal of play is, for whatever your aesthetic of play, there is a viable character that fulfills that goal of play better than a non-viable character. This is true even if your aesthetic of play is gonzo and you are happy to play comic relief, or if your aesthetic of play is casual and you just want to hang out with your friends and socialize. The only goal of play I can think of that is fulfilled by playing a non-viable character is if you actually want to experience and explore the frustration of failure and defeat, but that sort of thing doesn't require a bad character. Nor for that matter can you say, "I enjoy playing a character that has flaws and weaknesses so I need to play a non-viable character." There is nothing that prevents you from playing a viable character with flaws and weaknesses whether mechanical or imposed through your RP of the character (deliberately chosing less than optimal play in order to reinforce character traits).