I would be inclined to agree with you. The current mode is working sales wise, why slaughter the goose that is laying golden eggs.To be fair, just because you can play Frankensteined D&D doesn't really make them entirely compatible. I mean, I made a game that my group played very wonderful games for an entire decade that was a mash-up of 2e D&D and Call of Cthulhu - but I wouldn't call those two games compatible.
That said, I am now entirely convinced that 2e and 1e were not hard to mix, even if my group never did it.
And yet I still maintain that I believe that any new D&D for the foreseeable future will still count as 5e (with new swap-out options), and not 5.5 or 6e.