My interpretation of the ending:
I believe that after TTJ died so that the array would be destroyed and that Evil Bone could truly shatter, a portion of his soul/immortal essence/power seeped into the Heart-Guarding Scale and he's quietly cultivating there. There's definitely hope especially after the 500 year time skip, that eventually in a few hundred years he'll have regained enough power to be able to cultivate another human body for himself. I.E. cultivation novel style where after a few hundred years even animals can cultivate a human form. And since Li SuSu is still alive and thriving with their little daughter, it's no biggie for a Goddess like her to just wait a few more years (because let's be honest hundreds of years is like taking a nap in their eyes) before her hubby can join them again! <3
As far as I'm concerned this is a soon to be happy ending: the world's safe, they've got a kid, and the ML's not fully dead. Obviously it would've been better if they both walked away without any scrapes and bruises, but given the seriousness of the Devil God/destruction of the world, this ending is fine too. If anything, I wish we'd gotten a bit more closure when it came to the supports.