The third party D&D publishers have got a good 20+ year easy start, and one year of warning before that sweet deal goes away. If these people are good at what they do, they'll apply that know-how to keep producing great content. The ones that produce great content but can't adapt, and see no way forward other than to continue selling D&D compatible content under unfavorable license terms... well, like I keep saying, i don't think those exist. The venn diagram of "people who make good content" and "people who can not make content without the OGL" has no intersection in my opinion. If you can name a content producer that literally CAN'T make a book worth buying without the OGL, then I will confidently bet that their content sucks.