And while 5e has somewhat simplified this, everything you talk about is still relevant particularly if you are playing a game with feats and magic items and thus complicating your balance issues. And to the extent that you can take short cuts, those short cut techniques have been around since 1e days. Probably the absolutely best short cut in both cases is simply having a stock character of appropriate level to serve as a template for any character minor enough you don't want to stat them out.
I responded to this post earlier, but I was on a break at work so it was a bit rushed. I wanted to address this point specifically now that I have the time.
It's very different in 5e because the monster creation rules have shortcuts baked in. Compare the MM archmage to a high level wizard, or the assassin to a mid level rogue. All of the fluff has been pared away, leaving just what you're likely to need.
Compare that to a 3e NPC stat block. Everything is there - all feats, skill points, and gear.
Yes, you can do the same thing that 5e does with 3e and simplify it. But 3e never lead by example in that direction. I'm sure there were multiple DMs to whom the thought never occurred (as was the case for me back in the day).
5e, in the books, acknowledges that parties with magic items and the like are above par and therefore may require more to challenge them. The encounter guidelines are set up such that they create a challenging encounter for an inexperienced group that is not optimized, without feats or multiclassing. If that isn't your party, then you can ratchet up the difficulty as needed. It simply sets a safe baseline that you can use. Unless you do something completely absurd, like seeing that encounter A is too easy and then making encounter B deadly x10, you have the leeway to feel out the party's strength without accidentally TPKing them.
Which is quite different from the 3e encounter guidelines that assumed moderately optimized characters with useful gear of WBL. Non-optimized characters without WBL gear were under perpetual threat of TPK if you used the guidelines.
IMO, one of these serves as a far better instructive tool to new DMs than the other. Experienced DMs can get by without them (though I find them useful as a sanity check on difficulty).
Sure, in the case of a throwaway fighter who is just going to show up in a single encounter, using (or modifying) a stock stat block is the way to go. Even I did that back in the day. But if it's the boss, you don't want to use a stock block. And frankly, because the bonuses in the block often weren't explained, unless you had the system committed to memory modifying the block could be almost as much work as creating whole cloth. If I change feat X for Y, which of the numbers in the block change and by how much? It often required quite a bit of reverse engineering to arrive at where all the numbers came from.
Compare that to the table in the 5e DMG that simply tells me the numbers to use for a given CR. I'm not kidding when I say that I'm pretty sure I can use that to create a creature more quickly from scratch than I could modify an existing stat block in 3e.
So, to restate my original point, while I don't doubt that there were DMs whose experiences differed from mine, I found 3e exhausting to run. Though an experienced DM could work around those issues endemic to 3e, an inexperienced DM was guided down a path of spiraling workload that could cause burnout.