Rather than a flip "money," I will detail the cycle.
1) TSR (or WotC) puts out a new version of the rules, a set of core rulebooks, typically a Dungeon Masters Guide, a Monster Manual, and a Players Handbook.
2) Some additional content is pushed out, like a module or two.
3) Here is the danger part: Realizing that there are more Players than DMs, well, more money is to be made putting out content players want. Splatbooks of unlikely "prestige paths," or if you want to go old school, barbarian and cavalier and acrobat. Rather than having rules so flexible you could re-enact Conan the Barbarian or Sword and the Sorceror or Beastmaster, instead rules are carved out for an archetype of exactly that film or book.
4) The game unbalances due to new rules, and Pun-Pun the Kobold is born.
5) Players proceed to tear through every encounter with min-maxed ("munchkined") characters as per the hyper-optimized builds laid out in the splat books.
6) We must rebalance the game.
7) Go to Step 1.
This is part of the problem, not the whole of it, but ... I would not say it is insignificant.
#3 is more complex than I let on. It needs a few things to remedy it:
A) A focus on creating material for the DM, and most especially pre-built playable areas. A decent-sized city would cover an entire book, for example, as it would have several maps, scores of encounter areas, and just as many, if not more, fleshed-out NPCs with their own goals, as well as areas "just outside of town" to be worked with. Or take the Dungeon Dozen, but make it 1) serious, 2) strictly fantasy, 3) two hundred entries long.
B) An allowance for players to craft their characters, but not along the rails TSR or WotC or some d20 affiliate has made. Referring back to my example, Conan (the movie Conan) wasn't a "barbarian" class, he was a lot of fighter, with a little thief, a guy whose CON exceeded his STR, and he should be buildable by taking a little from this or that, some out of this skill tree, a feat or two here and there ... this eliminates the need for splat books.
One thing I don't get is why people keep buying new material. It doesn't make financial sense to me. Suppose you're a 4th ed. DM and have bought a bunch of supplements, and so have your players. Then 5th ed. comes around. Now you're supposed to throw all your books, and investment in them, in money to buy them and time to read them, and understand them, and make scenarios and campaigns and characters and whatever, in the bin and start fresh? Why?
I got the same question about Magic: the Gathering, which I play a lot, and Warhammer (which I mainly collected to paint the minis, though I played also). Why would you buy a new edition of a game every few years?
The answer I think is- you wouldn't. Which is to say, all those new editions are sold mainly to new players, who have no investment in the old editions and no attachement to their style of play or whatever.
Which in turn is to say that making a new edition of a big, expensive game like D&D every few years is takig a big, juicy dump on the players who bought the earlier edition, probably betting on the fact that they now have jobs and kids and no time for gaming.
So I understand there would be a pushback on that, and that's partly how I understand the OSR scene/ genre/ movement, whatever. My personal choice is to make my own games, and play them the way I want to play, and not care about D&D, past, present or future. It's free and creatively fulfilling (making your own game) so what's not to like?
1) TSR (or WotC) puts out a new version of the rules, a set of core rulebooks, typically a Dungeon Masters Guide, a Monster Manual, and a Players Handbook.
2) Some additional content is pushed out, like a module or two.
3) Here is the danger part: Realizing that there are more Players than DMs, well, more money is to be made putting out content players want. Splatbooks of unlikely "prestige paths," or if you want to go old school, barbarian and cavalier and acrobat. Rather than having rules so flexible you could re-enact Conan the Barbarian or Sword and the Sorceror or Beastmaster, instead rules are carved out for an archetype of exactly that film or book.
4) The game unbalances due to new rules, and Pun-Pun the Kobold is born.
5) Players proceed to tear through every encounter with min-maxed ("munchkined") characters as per the hyper-optimized builds laid out in the splat books.
6) We must rebalance the game.
7) Go to Step 1.
This is part of the problem, not the whole of it, but ... I would not say it is insignificant.