Seriously, what makes Perl Perl? If someone decides I'm ugly and beats my face to a pulp, and later I get a face transplant, am I still me? If my bones rot and are replaced with synthetics, at what point am I a different person? Syntax is just skin. Semantics are just bones. Neither is the soul of Perl, which rests in the realm of pragmatics.
Maybe it would help if you thought of Perl 6 as something more like Perl 16 or so. We're just trying to skip over the 20 years of deprecation cycles and dead ends it would take to evolve Perl 5 into Perl 16 piecemeal, even assuming that were culturally possible, which it really isn't. Plus I'm too Impatient to wait that long.
I've also seen what happens to other languages that change their name. They basically lose their branding, and have to start all over. I'm too Lazy to do all that work again. Plus there's a longstanding cultural assumption that major version numbers indicate incompatible changes, despite the recent trend for marketeers to pretend that great strides have been made when they haven't.
Another factor is that four letter words are in short supply, and we shouldn't use them up so quick, especially for things we really want to use four-letter words on. :-)
But overriding anything else is the fact that I think I have a moral claim on the name, and I want Perl 6 to be considered a better Perl than Perl 5. Call it Hubris if you like...