I think we've seen that people have been willing to work on D&D at WotC regardless of whether they got their start writing products for them.
The problem isn't people working on D&D, it's people
legally working on and supporting 5e. There's a lot of stuff the fans could do, and we just don't know about the legality.
I've said before that the OGL has two very different audiences: the publishers wanting to produce books for profit and the fans wanting produce books for fun. The former can muddle through with the 3e OGL (as they're a business and likely know a lawyer and have experience reading and interpreting the OGL) but the latter generally can't, or it's simply trickier.
No OGL to prevent 3PP is fine and WotC's prerogative (many would disagree about the benefits of the OGL, but in the end it's WotC's call), but nothing aimed at fans is problematic and puts them in a legal bind that not everyone will realize.
And supporting fan creations can be beneficial to WotC. For example, conversion documents for past adventures. That would be beneficial, as a list of monsters and advice for old modules could encourage people to buy PDFs for those off DriveThru RPG. But, currently, there's no solid guidelines for how should people present such a document and where the line is drawn.
Can you copy monsters? What about adding classes to monsters or making classed NPCs? If you can't copy monsters from the
Monster Manual, what about the Basic rules? Plus, new monsters. The
Monster Manual was awesome but there's a lot of absent monsters, which might be needed to convert content. However, the statblock of 5e monsters is part of WotC's trade dress, so that cannot be legally copied, even under the OGL. Any 3rd Party monster needs to have a different statblock set-up, which can be confusing and make the product harder to read.
Plus, at the very least, guidelines that tell fans what not to do saves WotC from earning potential negative publicity from having to shut down fan sites and projects.
And besides, there currently is an outlet for people to write D&D product... all the AL modules. Alphastream has done some really good work for them and is someone that could be in the pipeline for perhaps future employment. Independent OGL work is not a necessity.
Those have higher standards and require approval. There was an open call and the vast majority of applicants were rejected because they only do two each month and there's a decent pool of adventure writers already known from past RPGA seasons. Plus, a lot of authors do multiple modules in a year, so there might be only one or two slots free. That's some stiff competition.