How was the D&D IP not theirs?
Is an Orc, Dwarf or Elf an IP that's owned by TSR/WotC? Many, many things in D&D were just 'stolen' from others, which in turn 'stole' them from someone else. Only very specific implementations of D&D TSR/WotC could claim as their 'own'.
You could replace the word 'stolen' by 'inspired' to get the same point across, but this better represents the claim to said IP.
Imho D&D's main strength AND weakness is it's generic implementation. This easily allows people to make their own worlds or use existing worlds made by TSR/WotC.
I think one of the main issues at TSR/WotC was that many of their products that you kind of needed to run a campaign that you didn't think up yourself were not really profitable: Adventures, non-standard campaigns, non-standard source books, etc. That is pretty much the business reason for the OGL/SRD, letting others do the (for WotC) unprofitable stuff, while keeping D&D the main RPG for every imaginable product. We saw that when WotC licensed out Dragon and Dungeon magazine and eventually stopped that completely. And the very limited amount of Adventures for the 3E/3.5E run.
The 'problem' for WotC was that eventually the people making the 'unprofitable' also started making the profitable stuff, doing that better then WotC did, eventually moving to making their own core rule books, thus no longer needing the core D&D books. Pathfinder is probably the best example, but there are so many more. Monte Cook Games for example is another good example that eventually created such a positive following that it moved completely away from D&D and started making their own universes and game systems. The same goes for Green Ronin and quite a few others...
WotC didn't learn at all from 4E and tried to put the genie back into it's bottle with the OGL 1.1 shenanigans and they did that at a point of time when they were already getting an absolute crapton of negative press from their MtG side of the business. How management didn't see this coming a mile off I'm still unsure of. On one hand they rolled their decision back and apologized with the CC version of the SRD, on the other hand I'm waiting for the other shoe to drop...