I'm pretty sure the literal only reason Bart came back is because Bendis wanted to do YJ and he has a lot of pull. After that it was just deciding about how to do it, so it was clearly tacked on at the end of Flash War as a convenient place for it. It also explains why, despite having just come back, Bart was in no comics for like half a year -- they had the vehicle, but didn't have YJ ready to go yet so Bart just kind of sat around doing nothing for awhile.
The problem with any Earth 2 concept is it'll be one, maybe two comics that won't ever be able to explore any characters outside of a team setting or whoever is the Batman because they're possibly like to get a book called "Batman: Earth 2." You don't get to deal with any of the transition, you just plop in a timeskip and you're reading a book that is somehow even less relevant than the Titans and future generations already are, because now their wheels aren't even spinning in the relevant universe.
One of the utmost problems with the stagnation is page count. You don't fix that by adding another universe and more comics. Not to mention that the core universe would still be just that, the core universe. Events would run through it, major writers would be dedicated to it. It'd literally synonymous with being second tier because it's Earth 2. You get nothing of anything we're talking about. And I know you could call it something besides Earth 2, but whatever name you give it will set it apart as secondary to Earth Prime/Zero. A bunch of characters who still live in the shadow of their predecessors in the real universe.