prosfilaes
Adventurer
Frog God/Necromancer are lawyered up. They even have a judge (Clark Peterson) as a close ally and advisor. They're using the OGL to access certain vocabulary, they aren't using any trademarked terms or mimicking trade dress, and they know exactly what they're doing and how to do it. WotC isn't going to "come down on" Frog God.
TSR vs. Mayfair (1984) ended up with Mayfair having a perpetual, royalty-free license to use the AD&D trademark with the Role-aids line. It's pretty clear that TSR knew they didn't have a leg to stand on when they made that agreement, but I'm pretty sure the details weren't all public at the time, and the simple fact they sued Mayfair probably scared off a lot of people. (I wouldn't be surprised if the details only became public knowledge when TSR sued Mayfair in 1991 over a breach of contract, and intimidated Mayfair into selling the Role-aids line to TSR.)
IBM has awesome lawyers, yet SCO has kept SCO vs. IBM going for more then a decade. Can Hasbro lawyers convince Frog God that they are better making a sealed settlement that doesn't cost Frog God much instead of fighting for another few years (and fighting TSR is one of the things that killed GDW)? I don't doubt it. And that would enough to keep me from trying to release 5E supplements, and I suspect most companies would follow suit, instead of inviting a lawsuit.