Aging's world is peaceful, without consequence, free of urgency and responsibility. For basically Denji's entire life, and Asa's since the start of Part 2, the two have barely had a moment of peace between each other. Now they're here, and with the way Fujimoto writes things, the anticlimax that'd be them staying may well be the best possible ending for either of them.
At least, I hope they have an opportunity to work through some of their trauma together. Yoru's got a lot to think about too, now that Chainsaw Man's here, the violent solution is out the window, and she kinda seems more into the human him than Asa is these days.