It depends on what you mean by "designed". If Perl 5 had been designed the way Perl 6 is being designed, you might still be waiting for it. :-)
But to answer your first question last, I think that the designer of Perl 5 was short-sighted enough that it didn't even occur to him to ask himself whether Perl 5 was supposed to be the last rewrite. The guy designing Perl 6 is a little smarter. Fortunately, or unfortunately...
I can't recall anybody ever saying anything about the last rewrite in connection with Perl5.
Perl5 was neccesary to make it possible to extend Perl by using modules - in Perl1-4 that had to be done by compiling in the changes to the language leading to oraperl (with Oracle connection), ingperl (with Ingres connection) and so on.
Perl6 is neccesary because the core of Perl5 is so old (10 years) and extended that any changes risk failures. the language needed an update - just like the Perl4 -> Perl5 update.