The episode wants to be an Oscar-bait or something, i know it is a type of loss not that far from having a death in the family, to let a child go to have their own life...
But nobody moves out, it is just that Marge feels that Bart is not clingy anymore, and started forming opinions and not an "agreeing to any Mum activity machine" any longer.
It's hard to find the usual Simpsons entertainment in this that one usually pops one up on the screen for. It's all very artsy, maybe a Homer having a chilly hallucination marathon in the desert thing but with Marge, but the women edition of it prohibits it from being funny or something?..
I just had to see the very first frame of the Terminator 2 dream sequence, i immediately knew what's coming, and i said to myself no way, they are not having Marge as a flaming skelet---oh yes they do.