We know that some would at least like a share of the profits for successful products. One of the reasons to make the core systems open is so that 3PPs can throw out D&D related ideas and concepts, frequently without significant risk (the other being that it's legally dubious how much they could actually control). A lot of 3PP are just a side gig or creative expression for people, in many but not all, cases it's not their livelihood. WOTC can take a look at 3PPs that are successful and decide if they want to incorporate any of the concepts. Which, to tie it back to the thread if there were some supplements out there that had mythic fighters that were wildly popular I suspect we would see some movement in that direction.
But having 3PPs that extend D&D instead of actually building their own systems is likely a big part of D&Ds dominance. On the other hand don't forget that as much as D&D may be the 800 pound gorilla in the room it's still a pretty small room.