Call me a cynic, but I've always seen 4e's main reason to exist "there are too many people (who are not us) selling our game", so they had to change the core in order to be incompatible with previous non-wotc content, while at the same time changing the conditions to publish for it. 4e is to prevent more Paizo or Mutants & Masterminds cases.
I think the comparison that is most apt is "barnacles on a whale."
Sure, the barnacles get a leg up, but the whale doesn't really care. The whale is the size of six busses, what the hell does it care if some thumb-sized things use it to help themselves?
In other words, I would be quite surprised to find that 3rd party sales where anywhere NEAR threatening for WotC. They just don't have the size, the weight, the power, even collectively, to do that. D&D is such a behemoth that you just can't match it. You can do minor successes in your own way, but you can't really threaten D&D.
Not to mention that the GSL doesn't prevent this sort of thing from happening -- fair use says I can make a game with the exact same mechanics as 4e and just different trade dress and fluff and terms and sell it for a profit.
I don't think that 3rd party publishers were any reason for 4e to exist. I think WotC took the opportunity of 4e to refine some things they didn't like about the d20 STL, but to say it was the prime motive for 4e would seem to me to be vastly overstating the case.
But let's not forget the RPG market is that, a market. If you rant about how I screwed your childhood fantasies with my game, but buy every supplement I publish nevertheless, your vote counts as much as zero. Same way, if you love edition X and don't plan to buy any book for it, you are not "supporting" that edition, no matter how vehemently you claim to do on some internet message board.
I think WotC's message, all throughout the last two editions, has been remarkably open: "Speak, and we'll listen." They know D&D lives and dies on its community, and they have a vested interest in giving the community what it wants.
4e's changes were mostly, we were told, to address problems with 3e. Since not everyone seemed to have the problems with 3e that 4e supposedly fixed, there is some motivation to keep the problems with 4e front and center so that when 5e rolls around, they won't be ignored.
Not buying stuff might make 5e come sooner, but it's not going to make 4e go away. There are legitimate big fans of 4e, there are people who play 4e, there are people who will buy 4e books. 4e is here to stay for a while.
"I hate 4e. It's the worst game ever. Every supplement (which I bought) is worse than the previous, and I'll buy every further supplements they publish just to see how awfully written designed and conceived they are. With 3e I just bought the core books and never needed anything else, and it was way better than 4e" means to WotC: "you're doing it right"
Well, to any company with savvy market insight, what it really means is "I am loyal to the D&D brand." That's a hugely good thing for WotC, but it does have a breaking point. WotC has listened to the masses before, apparently ("halflings are too small"?), and they will probably continue to do so. Sales could always be better.
