I would even argue that the incompatibility of 3E and 3.5 is somewhat overstated. To be sure, 3.5 left a lot of 3PP works in a lurch, and from a "system mastery" standpoint, many builds and mechanical synergies were invalidated. But the core engine was still the same, and I daresay for most tables, 3E adventures could certainly still be played using 3.5 core rules with a minimum of fuss.
But then, cards on the table, I consider B/X, 1e, and 2e to be essentially compatible, at least at their core.