Most splat books add nothing to the core game, and frequently detract from it.
Some good early examples of this were the 1e AD&D Unearthed Arcana or the 'new tech' expansions to BattleTech that introduced clan tech, gauss rifles, and so forth. Neither made the game that they expanded on better, and unfortunately this was seldom immediately realized.
Often splatbooks violate principles of the game that made the original balanced or interesting. Often they add bloat without adding anything of value equivalent to the bloat that they add. Over time, the low hanging fruit is picked, and quality of a splatbook begins to decline, yet because the splatbook is canonical, a DM often feels pressure to accept it and include it in his game. Over time, enjoyment of the game decreases and it is often the case that the group doesn't realize its the splatbooks that are contributing to the problems. Both UA and Battletech's expansions are examples of a consistent pattern that shows up in splatbooks - power creep is a selling point.
The basic problem can be described as such:
a) The initial publication of a game surprises the publisher by creating a comparatively large revenue stream.
b) The company expands proportionately to their revenue stream sure that they have found a market.
c) Sales of the core book begin to taper off because the market is saturated.
d) To maintain the same level of revenue, an aggressive publishing schedule has to be put into place.
e) Initially this seems easy. There are lots of features that were left out of the core books, and demand and interest in your game is still high. But the further you go, the more niche your publications become, and the less obviously high quality ideas you have. When printing the core books, you were forced to cut everything but the content that had the highest value. Now, you find yourself stuffing things that wouldn't have made the cut into the book to reach a target page count.
f) Eventually, only hard core completionists are buying your splat books. Sales crash. Your over extended company makes big cut backs.
In general, I think a smart RPG company should not expect to have a steady revenue stream from a single product line and have that product line on a very long production cycle. Ideally, you have 3-4 product lines of equal worth that you shift focus between, and a very limited amount of support for the non-immediate product line. Those product lines can either be rules focused or they can be intellectual property focused. TSR originally had both - Top Secret, Boot Hill, Gamma World, D&D on one hand, and Dragonlance, FR, Planescape, Birthright, Ravenloft, etc. on the other hand. If they hadn't have been complete idiots, this should have worked. Even then, had it not been for the occult scare killing their main brands marketability, it probably would have worked. The amount of creativity TSR had pales any PnP company that has come after them. WotC has tried to solely be a rules focused company that doesn't produce IP. It's not really working for them, but they don't seem to realize why. Paizo is in the middle. They've got one core rules set, but their AP concept means they are actually getting a lot of their revenue from intellectual property creation in a way that WotC seems unable to master.
I would think you could reboot individual product lines every 12-15 years, and you know that your reboots are going to be 'fat' years and that revenue will taper off in 'lean' years. This is largely what I think traditional toy companies and other companies that recognize that their products are individually fads whose popularity will come and go do. Very few product lines enjoy a large degree of support year in and year out. TSR was obviously overextended in the mid 90's, simultaneously trying to support far too many settings and different sorts of splat books. White Wolf got overextended in the late 90's. WotC was obviously overextended in the 3.5 era. Paizo strikes me as being in the early stages of being overextended now.
Any game line in my opinion supports only a limited amount of non-intellectual property. You probably can get about 15-20 books out before you run out of ideas with wide appeal and use at the table. You probably ought to know what those books are from the beginning, stretch them out over say 6-7 years of publishing with most of that in a big early burst, and then 3-4 years in introduce some completely different style of content.
I'll be disappointed if there is ever a truly new edition of D&D after this. It would show me that they just don't get it. Basically, I think 5e should be in revision forever. Rules are not that important, and certainly the utility of a rule book is something with a diminishing margin of returns. The 5001st through 5300th page of rules is never going to be as useful as the 1st through 300th. Fantasy is riding a good wave, but in three or four more years the exciting news from WotC better not be, "Jungle Themed Supplement for D&D Next as part of our new exciting Environments Line", but something like "WotC aquires rights for the Parasol Protectorate. Plans brand new Steampunk themed game."