Do you have examples of libraries which change their namespaces in some form on each major release? I know Guzzle did it (they also moved to a new repo) with their last major, and I believe the Amazon SDK did too, but those are the only examples I can think of right away.
I personally don't think versioned namespaces is the right way to go. I don't see it addressing any issue, in fact I only see it opening the door to more issues. Sure, in the CMS users would absolutely love it if you could run FW v1, FW v2, CMS v3, and CMS v4 all in the same request cycle (I'm sure they'd love their Mambots to still work today too), but I see code bloat and really bad code structures if you even give developers the option to do this. It just seems like a majorly bad idea to build an application environment or enable developers to build one where one component is using Registry 1.4 and another component 2.0. I'll concede the one thing that versioned namespaces enables is easier development of a CMS extension for two major version branches, but is that really worth making this change?