Here's a funny thing, to me at least: gamers, myself included, have longed for the quote "good ole days", but the way it is now reminds me a lot of late 80s/early 90s. You have multiple editions of "The Worlds Most Famous RPG" currently in play with various amounts of support, a different "main game" for each genre (gamma world for post apocalyptic scifi, spycraft for espionage, etc), and a bunch of games on the shelf that you've never heard of because there are so many different publishers from all over the world.
So, in effect, we have the "ole days". Just a little older and maybe not "good" depending on how you look at it.
The thing is, this takes care of the whole "D&D represents the industry" concept. Just like I'd wager most of us did, a new generation will try the flashy, big box products, whether it be Pathfinder or 4E. They will decide whether they like or not, but if they like the concept of RPGs, they'll try other ones. For us, it was Ars Magica and WoD. For them, it'll be Risus and All Flesh.
If this is anything, it is diversification, but I don't personally like that term for this. I'd call it normalization, because d20 brought a sort of unification that was never seen before, and thus wasn't normal. No matter what you wanted, there were d20 rules for it. Now, we go back to different games for different games.
I don't agree with the idea that the RPG market is only big enough for one game or version of a game. This isn't Middle Earth, and there is not One Game to Rule Them All. Pathfinder and 4E are not, in terms of the market, that different a situation from 1e and 2e in the late 80s. In fact, it's better because we have more support for both. This is a problem for WoTC in terms of sales, but in the terms of the industry and the hobby it's a good thing. More people playing different games means more players and more product.
As for 5e being a disappointment...You can't call it a disappointment because it doesn't exist yet. It's like being disappointed that your personal time machine takes 1.21 gigawatts when you don't even have one. That said, I would lay fair odds that WotC will not find the sort of success they'd like from that product. The truth is that I doubt there is anything they make to approach the level of success d20 enjoyed, short of purchasing Paizo and merging the product lines in to an old-school Basic and Experts line while producing dual-stat source books.