It's also good, because it means they don't even have to make a complicated explanation for them to return later.

If, and when they finish figuring out where to go next, and supernatural finally returns, they can use that.

(They've already said on many occasions they're just looking for a worthy story to tell in order to bring it back)

If Dean can just waltz from his universe's heaven to an ENTIRELY DIFFERENT universe's earth that easily without help, he can probably show Sam how to as well.

All you need is to come up with a reason for them to want to re-enter the fray.

Some world/universe-ending threat, (Or even multiverse-ending threat. Since we have to top God himself somehow) that seems serious enough for them to be like "it's time for one last hunt" and hop in Baby. With, or without Jack's help.

I mean, Dean could always call his ex... aka, ya know, GOD'S SISTER 😂.

They do have that whole telepathic, or soul-binding, or whatever the hell connection with each other, so he could probably her through it somehow. Kinda like how that one psychic dude was able to send Dean and Bobby a telepathic message about where Azazel was keeping them. Or how Harry and Voldemort can send messages if they focus hard enough, like when Voldemort tricked Harry into going to the ministry to "save Sirius".