To suppose that Perl 5 will be actively developed 8 years from now is to greatly underestimate the importance of an innocuous little thing called the version number.
I cannot imagine anybody seriously using an 8 year old obsolete version of anything and not being in the tiniest of minorities. Who uses Windows 95 anymore? Linux 2.0 or older? Java 1.0? PHP 3? Perl 4? Yes, old versions do get used, but only in special circumstances.
Sure, Perl 4 was good enough for what it was, but then along came Perl 5 which was also good enough for that, and more. And today Perl 5 is good enough for a great number of things, and Perl 6 will be good enough for all them, and even more again.
If Perl 6 were renamed to something other than Perl, then I could see a future for Perl 5, but if there's a bigger number available, it will get all the attention.