Yes. Yes it is. For most of D&D's history, such a character is represented as an NPC, traditionally a "0-level" NPC. D&D's core design concept for the party is a group of quasi-protagonistic tomb-robbers...err...adventurers acting like a fantasy special ops team. Character Level is a generalized measure of a PC's ability to stomp on the bad guys and defy traps and other hazards. The centrality of the tomb-robbing part has varied a bit over the years, but the adventure part hasn't.
This reasoning doesn't satisfy me one bit. You begin to touch on why immediately, though:
I realize that people play D&D in a wide variety of playstyles and with a wide variety of motives.
And this is exactly why. D&D 5e wants allow or support multiple play styles. That's good, in my mind. While the majority certainly doesn't need completely combat inept characters, I think that the majority would enjoy shifting the 3/3/3 balance around from time to time. Even if one person from every group does it at some point, that means that every group would be affected by that optional rule.
That's not insignificant. For groups like mine, it's a requirement. But, hey, I'm not going to switch over to it as my main game, anyways. However, I might play in it when my brother decides to run a game, or occasionally run it myself as a one-shot. To that end, having a character that isn't forced to silo his abilities is very important. And, again, it's not just important to me. If one person from every group shifts that 3/3/3 balance just once (with permission from the DM and possibly other players), it would have an effect on every group. I imagine this would be true for at least 50% of groups at some point.
However, the concept of Fantasy High-Adventure is pretty central to the design of the game. I can't think of any edition (barring Dragon articles or the near-infinite splat material) where playing a "I don't fight or contribute to fights" is an easily workable concept (maybe a wizard who refuses to cast spells?).
You know those NPCs you mentioned earlier? Sometimes people think they'd be fun to play. Sages, inventors, courtiers, etc. How many characters from Game of Thrones (or Song of Ice and Fire) look like they'd be fun to play? To me, a lot of them. Sure, they include combatants (Jon Snow, Eddard, Jaime, Bronn, Jorah), but they also include people who are certainly not combatants (the Spider, Catelyn, Daenerys, Samwell, Maester Luwin).
People play the game in different ways. I'm not asking to change the default assumption. And asking for support for people who want to change the default assumption. And I think that's reasonable, when it could be as easy as "since Feats [combat] and Talents [non-combat] run on similar tracks, just swap your Feats for Talents." That's easy, simple, and intuitive, and it's optional. The baseline is still 3/3/3.
If you and your group are not interested in that...then, and I mean this earnestly... maybe this isn't the game you are looking for. (FATE, Burning Wheel, I'm sure there are others...) I don't suggest that lightly, either. I'd love to have a FATE group going, but AFAICT, I'm the only one in my area interested. However, if your group is interested in going that far afield of the D&D script, you might want to consider it.
Like I said, I almost assuredly won't be switching to it, regardless. I still have a vested interest for when I play in it, and also for the direction that the game is taking. If they do the same thing with multiclassing that I think they should, then I might look into it.
However, trust me on this, games like Fate and Burning Wheel aren't my style. Just because I don't feel like every character should be inherently wed to all three pillars (not just combat), it doesn't mean that I enjoy more dramatist games. I don't, when it comes to fantasy games. When it comes to scratching the non-3/3/3 itch, though, as I said, we just use my RPG.
Okay so...You don't want to swap a combat specialty/feat for more OoC/Background stuff, but then you do?
I said that if they're not on the same schedule, then I'll have an objection. If they're both every level, of both every other level, etc., I'd be okay with it.
Is it just the idea they might not be called the same thing? If they are separated into Columns A and B, then adding items to Column B that say "get more from Column A" has got to be one of the easiest houserules I can think of (if it isn't core from the get go.) But then you seem to be objecting to that as "an appropriate answer"...
I'm not trying to be combative when I say this, but did you jump into this conversation and disagree with me without reading the back and forth I had? I said, rather explicitly, that I'm okay with them (combat abilities and non-combat abilities) being called different things, as long as they're on the same schedule.
I think the critical thing about this is that the relative desirability of abilities between the three pillars varies a lot between campaigns/characters, making it very difficult to balance between the pillars.*
Which is why I'm in active support of 3/3/3 being the baseline.
However, the combat stuff is always desirable to a large portion of the audience ("my character's survival is at stake!").
This is why I'm in active dissent as 3/3/3 being mandatory. In my group, the combat stuff isn't always survivable. And, yes, I know you said "to a large portion of the audience." But, the reason that I'm invested in this point, is that I'm part of a group that has the same take on it. My players have complained
when getting automatic hit points at every level in 3.5. It didn't fit what they wanted from the game.
I think giving the kind of support I'm asking for should be easy enough. I really do. And it doesn't change that baseline assumption at all.
Putting them both in the same pile means that many players will feel obliged to always select the maximum-value Combat choices, and ignore OoC stuff ("trap options", etc.) In this way, a lack of siloing decreases viable character concepts for a large segment of the audience, turning every character into a combat machine. (I know that's not realistic behavior for humans...but then D&D PCs are pretty far from realistic anyway.)
If you make Feats = Combat, and Talents = Non-Combat, and they run on similar tracks, and you make an optional rule that you can switch one for the other, you've essentially made them into one pile. That's fine with me. I want that. You've basically done the same thing as labeling them "Combat Feats" and "Non-Combat Feats." Make them separated, sure. Assume that you're getting an equal number of each, sure. The baseline is 3/3/3. But, let people swap the moving parts around to fit their campaign.
I also might disagree with your use of "viable", since I'm guessing that heavily depends on the type of campaign you run.
Because of the "survivability trump", I further think that these exchanges should be one-way. That is, you should be not be able to trade RP or Exploration abilities for increased Combat effectiveness. Doing so makes it very difficult to produce a reasonably predictable game at all.
Label the option as such. Make people informed. Don't force your style, though, when it should be easy enough to allow them their own. I'm in strong support of saying "taking this will lead to these results." Awesome, that's great to include. But, I'd really rather not hear "you can't take this because it would lead to the results you're looking for." That's exactly what I don't want to see, you know?
Able to punch more/harder? No? Must be just me... As always, play what you like