I am aware that it’s the pashtus, and of course we don’t pasken that way, but you can’t deny that R’ Moshe considered it a possibility, although I don’t understand it either.
It’s fair to say that paskening like the M”B is not an absolute (although the Chazon Ish makes it sound as if it is).
I do not think, though, that you’re correct that the reason the M”B is so widely accepted is merely convenience. It has to do with the author.