Often the concept of game systems getting bigger - more rule books, more supplements, more adventurers, more everything - is presented in pretty malicious terms. The companies want dependent customers, so they keep out a steady supply of supplements that you need, to keep you dependent on them.
While this can be true it's not the only model I see. I see this one a lot:
1) Game comes out. People buy it and like it.
2) People clamor for more support for the game. Authors and publishers and fans alike want to add their own stuff to the game. ("I like GURPS but it needs ninjas. Here's a book on ninjas.") If it doesn't get this support, it probably dies somehere along the line. If it does, it might be fan demand ("Write a book on ninjas!") or authorial ideals ("I love this game, but it needs ninjas!") Or it might just be a fanzine, or a fan publication, or a fan website. People want support for their games, and either buy it or make it.
3) Eventually there is so much stuff out there that people feel like it's too much, problems with the system's design choices are exposed, and want to try starting over with the basics. A new version comes out. Start over at #1, if it's done well.
You can try to dodge around this a few ways, and there are good ways to do it (supplements are useful but truly optional for play) and less good ways to do it (supplements are critical and non-optional for any kind of play in the larger community). There are good reboots (new version fixes nagging problems in the old one) and bad reboots (new version creates new problems, or serves only to replace something that didn't need replacing). Sometimes it works, sometimes it doesn't.
That's speaking as someone who writes supplements and buys them. I write stuff I think the game could really use, and I hope to sell them to people who think the same thing ("Yes, GURPS Dungeon Fantasy really does need ninjas!") It's part of the life cycle of games, in my opinion, and it's healthy rather than malicious.