Perl 6 will have a Perl 5 compatibility mode. Yes, it's a more ambitious effort and it's a loose analogy. However, I don't think there's a need to have both Perl 5 and 6. It's meant as an evolution to solve the same kinds of problems. Perl 5 came back to life after it became clear Perl 6 was going to take even longer, and it's adding some of the improvements from Perl 6.

Anyway, the observation is to note how long it takes to get people "upgrade" when "breaking" changes are made to languages.