Spitballing, but 3PP likely affects churn rate more than acquisition. A lot of 3PP target GMs more than players. If WotC retains a GM because 3PP covers a topic WotC doesn't, it retains that GM plus all of that GMs players. Some portion of those players then become DnD GMs and the cycle continues. Is that enough to impact WotCs bottom line? They clearly think not.
This is one way to look at it, but at the time it was designed, WotC really didn't see it that way. Ryan Dancey (the man behind the OGL) explained the rationale behind the OGL by saying that TSR had started competing with itself by releasing too many supplementary products (adventures, add-on rules, settings etc.). One benefit of the OGL for WotC would be off-loading the supplementary products to smaller third-party groups, and since all of these would still need the core rules to be used, D&D would benefit from being the biggest fish that uses the OGL while allowing more 3PPs to create stuff within its ecosystem.
What Ryan Dancey likely did NOT anticipate was where we are now where multiple people are using the OGL to create D&D clones that people can buy and play without any reference to D&D itself as currently in print, however. Which is exactly why the OSRIC guy, who's name slips my mind, had to deal with hasslin' from WOTC at the time, because that wasn't at all what they wanted the OGL to do. The whole existence of the OSR who ignores 5e and in fact disparages it in many instances was not the intent of the OGL at all. I can imagine why WOTC isn't thrilled with the actual result as opposed to the theoretical one.
OSRIC gave birth to the OSR movement, which WotC has been influenced by and hired designers from that scene, all the while energizing a group of players who had largely already moved on to other games or had just gone back to playing their tattered old copies of TSR games. It also kept people playing what was functionally D&D during a time when 4E had driven a lot of people away, which made it easy for people like me to come back when the 5E ruleset was more to my liking.
I find it difficult to argue that the OGL caused D&D / WotC harm when you look at the market and how dominant D&D is
There is no "WotC." There are people who work there. This all feels very much like two or three top execs who don't get the world of open source having a knee jerk reaction and the power to not listen to arguments to the contrary.
Those execs are ruining it for the rest of us.
That's what execs do.
