Perhaps I am displaying my ignorance here, but I am wondering what WotC gets out of making a 5e OGL? Is there any incentive for them to release it, other than making some fans happy?
The benefit is arguable to some, but clear to others. All its benefits are indirect but still there.
1) Filling the niches that wizards leaves behind. Not everyone is playing the campaigns that Wizards is selling (for many reasons). Some are running 5e compatible adventures like the
one from my companyhttp://www.drivethrurpg.com/product...m-Tyrkaven-5e?affiliate_id=231528&src=ENWorld, or Goodman Games or Frog God. Others are using old adventures and converting them over and others are taking pathfinder modules and converting them over. Each of these options is less preferable to new GMs than the previous, since each successive option requires more work on the GMs part. Compatible modules require only the additional work of finding out about the company. Others require finding out about it plus conversion. But help for a new GM means a new group that would not have otherwise existed or an existing group that would have disbanded due to GM burnout. If WotC isn't fulfilling that niche, a compatible publisher can do the job.
2) More sale of their books. Like I said above, a new GM means a group that didn't would not have existed otherwise. So that will probably include at least 1 player that had not bought the book previously. Lack of support for that GM means a loss of that sale. If Wizards went full Paizo and made all of their (future?) Core RPG books open for developers to tinker with, there will be more options for it, options that Wizards never thought of. Options that might be the reason that an existing fan bought the book instead of just passing on it.
3) More people with system mastery. This one was told to me by someone at Wizards. Compatible publishers are a training ground for new developers. You need experience to get better. And writing for that compatible publisher means that Wizards can hire people that are already experienced with the system, a level of experience they would not otherwise have. Sure, someone can be good writing for their home game about stuff they like, but when you have to write 10 magic items under a theme you had not thought of before all on a deadline, you learn more.
4) General good will. Anything open to anyone tends to have a halo effect in the same way that Gary over at Black Diamond Games
talked about indy games. People that are complaining about a lack of a license ... stop complaining about that. People complaining less tends to mean more good feelings and companies want their products to be associated with good feelings, because they sell more.
You can argue the size of these benefits, they are real benefits that the parent company gets.
