No this is the worst thing. And all other editions did better by not making them use the same rules.
AD&D has NPCs and PCs using the same rules, hence the need to differentiate Non-Players Characters from Player Characters. In fact certain characters bounce back and forth between PC and NPC, both IME and in what I've read of Gygax's table.
5E even has NPC-only subclasses like Oathbreaker and Death Cleric. That doesn't mean you can't let players play them anyway, but what you cannot do is argue that 5E NPCs are not allowed to have class levels. (Generic you, not you MonsterEnvy specifically.)
However. What you don't need to do in AD&D or any other game is show your work for NPCs. I've never played 3E except via CRPG, but from what I remember of ToEE there are long feat chains to acquire certain traits. As a DM you can skip to the end and just write down the end result. The same thing applies to monster equipment: you don't need to figure out how much their plate armor cost or who they bought it from or how long it took to forge. If it doesn't matter, just say they have plate armor, done.
You can even do this for PCs in order to get new players into the game faster. "Okay, you're a fighter with plate armor and a sword. You've been living as a hermit in the woods for the past 5 years, trying to figure out where to act on your secret knowledge, that your country, Ulm, and its arch-enemy Hinnom are secretly both ruled by the same cabal of mind flayers. You know some other stuff like languages but we can figure that out at an appropriate moment during play--just ask me, 'hey, do I know that language?' For now everyone is waiting for us--let's start play!"
www.dndbeyond.com
