In fact, Perl even had the tools to break backwards compatibility baked in from the v5 days.
I agree that Perl 6 is why perl died, but I think the thing that really killed it is what you mentioned. It was a completely different language that spend over a decade with a release date of "soon".
Who wants to work on a language that isn't being worked on because the next thing is AND where from what you know of the next thing everything will be a complete rewrite.