There is the
obvious issue with backwards compatibility there,
although that can be overcome with work. The other major issue is that
no WYSIWYG system is likely to be as powerful as wikitext, or as quick
and easy to use (for experienced users). WYSIWYG is certainly much
easier for new users, but once you are familiar with wikitext, it is
generally better.
Actually, I don't think these are even the worst problem. The worst problem is
that the wysiwyg editor would have to cover the full range of what wikitext can
do now
That's what I mean by "no WYSIWYG system is likely to be as powerful
as wikitext".