Do I think that someone other than Sakurai can competently design Smash-style movesets? Yes, absolutely. But inevitably their style and process for doing so is going to be very different from Sakurai's and maybe it's not best to force them into doing it within the confines of what Sakurai has already built up. It could end up ok, but at best it's suboptimal.
I mean, if a port is what they really want to do and Sakurai won't agree to work on it, then they will get someone else to proceed with their plan; I don't think this potential issue would stop them from making a port, again if that's what they really wanted to do. But I raise the point to indicate a potential risk in terms of the probable quality of content that would hypothetically be added to a non-Sakurai port. And to indicate that using a port as a sort of test drive with a new director may not be a viable motivation for doing a port, even if it doesn't turn out to be a technical impediment.
"The characters are all designed as if they're taken straight from their games": representing characters accurately, making them feel like themselves in Smash is the difficulty, not the ease. There is art and science to this kind of cross-genre translation.