All Canon has to do to create native super telephotos is build the converter into the existing EF and add an additional control ring. Presto, native RF.
So you are basically saying that Canon is lying about the advantages that come with the RF mount.
There's nothing optically wrong with the existing EF supertelephotos,...
Not at all, but does that mean that no improvements are possible? To take your statement to the logical conclusion - is Canon shouldn't be releasing any more improved versions?
... they're not going to get smaller, and they're not going to get lighter (due to a RF mount change).
I don't really know for a fact as there aren't enough RF lens to prove one way or another. Perhaps you can show us how you came to that conclusion.