I don't think it was just the backwards incompatibility on its own. I think perl would have had a much better chance if perl 6 had been ready sooner. The long period of time between perl 5 development stopping and perl 6/Raku being ready for production made the transition much, much rougher than if perl 6 was ready from the beginning, or if perl 5 development had continued, and moved closer to what perl 6 would be while perl 6 was still in the works.