While 5.1 is definitely a bit out of date (although there definitely are still apps using 5.1), 5.2 is very much still used in production by very many applications, and I believe actually still has a significantly higher install base than 5.3
And as VP mentioned, since there doesn't appear to be any “backward compatible” way of introducing namespaces without breaking compatibility, using namespaces for Qcodo-based classes will likely have to wait.
(Although, if there are any thoughts / suggestions on how to approach this without breaking BC, I'd be more than happy to hear them.)
Now on the other hand, if you want to support namespaces for your code-generated classes (ORM) for your own application, that could likely be implemented relatively easily by simply updating the codegen templates. Just a thought.