Its not about not "letting" Grant Morrison make a 4-6 long super-narrative. Its that Grant Morrison didn't have that kind of epic left in him to tell. Not for Superman, not for any superhero, apparently. People need to stop taking Grant's decision to stop monthlies and blaming it on DC.
And not even Grant Morrison could have fleshed out 1938 to Byrne to the pre-Flashpoint and made it all canon. You're asking the impossible. Superman continuity and Batman continuity are completely different animals, he couldn't have done for Superman what he did for Batman in this regard. The only possible way to even begin to try and make it work would be to actually made Superman arrive in 1938 and have the character be 74 years old in the present day. I know some were hoping for that when the relaunch was announced, but I thought then and still do think its a terrible idea.