If the OGL "failed" as an Open Source development project, as Mearls claimed, the fault for that lies entirely with WotC. There must be some sort of central authority responsible for incorporating the best ideas into the core and constantly redefining the "official" rules.
There were quite a few amazingly cool bits in Unearthed Arcana that I really expected to see folded in, such as Fractional BAB and Magic Rating, both of which really made multi-classing viable (while softening some of the spikiness of inflated saves and lagging BAB / caster level).
I wonder how much of that sort of thing would have made it into a theoretical 4th edition based off of the 3.X engine.