For the fourth or fifth time in this thread - telling us to ignore a rule because it was unworkable supports the point that the rule was no good.
Of course in real life people would often house rule and just make things up and not follow the process - like in editions prior and editions since - because the rule was bad.
Again, nothing about the guidelines for adjusting monsters was unworkable. They are really good rules and I've used them a lot. It's great to have all the consequences of adjusting HD or level set out for you. It's great to have guidelines for estimating CR. All of those were good things. They weren't bad rules.
Your argument seems to be that they were "unworkable" because you had to use them, and because you had to use them they were bad because they were time consuming.
But you didn't have to use them. You only used them when you needed to.
a) You could always build encounters out of unmodified monsters drawn from published sources like monster manuals, just as you had in prior editions of the game. You just just write
three orcs, 1 ogre, and be done. And if you did that, and you weren't confident of your skills, you had at least an estimation of how difficult that encounter would be which is not something you had in a prior edition. But if it is burdensome to make a bunch of customized monsters for every encounter, why are you doing it? You brought that on yourself. Nothing forced you to do that.
b) You could always choose not to stat out any NPC for which you were unlikely to need a full stat block. You could always write, "Jalob Sharp, Tout (Rog3)" or "Avard Haysworth, Mayor (Expert7)", figuring out hit points or skill bonuses or whatever if and only if it came up. There is no rule against that technique and it certainly wasn't invented by editions after 3e or deprecated by 3e.
c) You could always work backwards to the precise stat block starting from some assumptions. If there is one take away that anyone reading my response should remember it is
there is no such thing as an illegal stat block. No stat block you could make up breaks the rules. You are allowed to make up any stat block you want. Then, if you have a desire to do so, you can always work backwards to what the fully detailed stat block looks like in order to justify all your numbers. Of course this is easier if your stat block conveniently looks a lot like a standard stat block with few numerical deviations so that reconciling everything is less of a headache, but if your starting point was plus or minus one or two points off, that's not a big deal, and can be if you feel the need to conform for some reason always be justified using the rules. There are no limits on what you are allowed to create as a DM. There are tools there to help you create highly detailed balanced foes, and that's a good thing. But they are their to solve problems, and not create them.
d) Most adjustments were actually very easy to accomplish, especially if your goal is to just generate a quick stat block for combat. The things that took a lot of time were allocating skill points, detailed spell lists, and detailed equipment lists - the same things that actually take a lot of time when making any D&D character. But, for many foes, all that was pretty much irrelevant. For better or worse, most skills were completely irrelevant to combat. You don't need to figure out what a dinosaur's exact skill allocation is, any more than you needed to figure out the NWP's of an NPC in 2e. You don't need to assign it spells or equipment. Most adjustments to a monster didn't adjust the monsters spell lists. Combat was only going to last about 3 rounds anyway, so why do you need to know more than 5 or so spells that an opponent has available? Why can't you just pick plausible spells should you discover you need to do so? And why do you need to worry about all that equipment anyway? Remember, it's just
suggested wealth by level, and it's certainly not that important to building an NPC to make sure you have the
suggested wealth by level exactly right to produce some hypothetical suggested CR for an NPC. And if the encounter went in ways you didn't expect, it's easy to just calculate the foes 'Sense Motive' or 'Bluff' bonus on the fly. You aren't breaking any rules even if rules there are to do so. You can always use the rules to work out the rest of the details later. I can assign a +12 sense motive bonus and work out later where the rest of the skill points were spent, if I need to.
e) You can always reuse your own work. Once you've created a 7th level Wizard, all 7th level Wizards can be pretty darn similar. Once you have a 9th level "assassin" all similarly skilled assassins can be similarly skilled. Yes, you can lavish time creating important reoccuring NPCs, but foes that are only going to last 3-5 rounds before dying and being never revisited don't need the same level of customization. And again, there is no such thing as an illegal stat block.
Now I'm sure a lot of people had problems with the system. One sure fire way to have a lot of problems with the system is play it outside of its built in assumptions, but then not treat the system as a set of guidelines based on built in assumptions. All those guidelines make explicit what assumptions that they are meant to work under. The more you departed from the assumptions of the guidelines, the more you needed to treat them as guidelines. Yet I know there were people out there who routinely broke the assumptions of the system as if those assumptions were NOT "rules" but then who played with the rest of the system as if they were "rules" that couldn't be broken. One common mistake I've heard about several times on the boards is attempting to make the majority of encounters with a single opponent, possessing PC classes, and requiring a lot of equipment in order to hit their hypothetical CR. The system is certainly not built on the assumption that every encounter is with a single PC classed foe. But that would have broken as a design pattern in every prior version of the game as well, and probably would break in more recent ones.
And it's always been hard in every edition of the game to actually make a single foe challenging simply because of the action economy. That's not a trouble new to 3e or that went away in new rule sets.
Finally, you seem to be making this logical fallacy that if I didn't exactly follow the guidelines all the time, that this must mean that they are bad guidelines. But that's not true. They are really good guidelines and the system for modifying monsters is a really good one that empowered a lot of creativity. It's not like 3e invented the process of making novel monsters or adjusting the HD of monsters in the monster manual. I mean I had 18HD manticores way back in 1989. All 3e did was call it out and empower it and provide really flexible tools for achieving all sorts of novelty. But it's not the fault of the system that players would pick up something like Rappan Athuk or Return to the Temple of Elemental Evil, with the all the highly complex detailed stat blocks of customized monsters and think that it was a requirement for them as amateur DMs to do that sort of thing for every encounter. If you acted like that, you can't blame the system. You brought that on yourself. You could have just used the monster manual and created complexity through novel combinations of monsters. You didn't have to adjust the HD of every monster, add a template, and throw on 6 PC class levels for every single encounter. That's on you.