I don't see why people weren't able to have mature discussions about this in the first place. Surely most of us are adults here?

Maybe not.

Anyway, some people seem to think that there is a lot of evidence to support the theory that Rinoa actually goes on to become Ultimecia.
If you do a google search for the theory you'll find their reasons.
To be honest I think that they're pretty far fetched and don't really follow but people like to persist with the theory because they feel it patches an otherwise gaping hole.

I don't think it's a gaping hole, more of a "have whatever you want happen" situation.

Square are great like that and I think the plot to FF8 is complete and makes sense without any R=U.