I always cringe when I hear this battle cry. Isn’t that sentiment exactly what set the trajectory for the Perl 6 effort? Maybe it’s just been so long that people have forgotten.
But that is precisely how Perl 6 became such a amazingly long trek: once you remove the constraint of staying compatible, everything is suddenly, potentially, up for reconsideration. Then when you start changing things, you discover that changes in one part of the language also affect several other, remote parts of the language. So it starts with the simple desire to fix a handful of obvious problems in obvious ways… and spirals out as you make changes, and further still as you make changes in response to your changes, ever further and further.
At that point, it is exceedingly likely that the project will fizzle out before it ever comes to any fruition. But even if you have the perseverance, you face an uphill battle: unless your project has the community’s implicit blessing as the successor (as Perl 6 does, due to Larry’s presence), it is likely to simply slip into oblivion… the way Kurila did.
So yes: backcompat is holding us back… the same way that gravity is. It keeps us from floating away untethered.
Note that I’m not saying it doesn’t really hold us back. I’d love to travel to space easily, too! I still await Perl 6, as well.
But what I think, every time someone proposes to throw off the shackles of backcompat and go for it, is that we already have one Perl 6 – we don’t need another.