The first claim is pretty subjective, but I'll stand by it. Just look at the new classes in Old School Essentials Advanced Fantasy, then pop over to d20PFSRD.com and look at some of the third-party classes or class archetypes and imagine them simplified and available right next to them. It's a gold mine, it's free real estate.
Second claim, any number of warlock and/or psionic systems for OSE or LL have been adapted from 3.X or 5e; I'm particularly fond of Timothy Brannon's work for The Other Side Publishing, which has elements of both the Pathfinder Witch and the 5e Warlock. Tieflings and dragonborn as classes like elf and dwarf, magi and warforged. Want to say that Barrel Rider is the standout here, CrossPlanes, Necrotic Gnome themselves.
And maybe I'm biased here, because "The OSR" frequently complains that WotC/Paizo D&D content is "too weird" and one of my biggest complaints-- even while 3.X was still in print-- was that it was never weird enough, never as weird as Mystara and Hollow World and Spelljammer and Dark Sun. People are injecting absolutely any- and everything with the OGL license on it into the framework of the B/X rules and it is glorious.