I will play in such a game, and steal the rules I like. I'll even pay for the books, eventually. (I have another expensive book that's a higher priority for me right now.) But I'm still happier to run my OSR mish-mash. The killer feature, for me, is the simple game prep.
You can argue for and against certain rules, and you can reasonably assert that 1st edition AD&D was a disorganized mass of rules and odd exceptions, but for the most part, those rules did not slow down game prep or character creation once you got them under your belt. Not so for, say, 3rd edition, where the concept of a character "build" really took flight. Creating a character, NPC or player, takes a lot of time in 3.x, due to the need to worry about where to spend your skill points. 5e has reduced the skill list dramatically, and added "backgrounds" which allow you to customize your character in a simple and reasonable way, but it's still faster using the old rules.
This is important to me because (as my blogging record of late might indicate) I don't have a lot of free time, and I don't want to spend too much of that doing game prep. I like having a reasonable amount of detail in my game, but only in the areas I choose; I don't want to have to include game-mechanic complexity in my prep time, because I want to focus on character complexity, or the details of a dungeon map. Rolling 3d6 six times, picking a character class, selecting equipment and (possibly) a spell list is still faster. Especially when done during gameplay. And most monsters only need a single roll to determine hit points. At the end of the day (literally), I want something that isn't going to bog me down, and old-school prep fits that bill nicely. cheers, Adam