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Even in-universe, it doesn't actually seem to be really understood exactly what happens to them either, as far as I can tell.

After all, the following is said:

Goetic demons dwell on no plane of existence known to mages, but can manifest in Twilight as well as in the Material World. There are only three ways of destroying a Goetic demon: a successful abjuration by its summoner, raw magical force, or a solution to the emotional problem that originally birthed the demon.

So, on what happens to them when they cannot manifest on Earth anymore?

They definitely do continue to exist, even if not on Earth or in the Twilight, but do so in an unknown plane that - given it's not known about - cannot be accessed or apparently even learned about by a Mage.

Which would be why the information on it is so vague.

Then there's also this little fact too, that merits a mention -

If the host of a Goetic demon dies, the creature is free to roam Astral Space and the Material World of its own volition.

In this case, it doesn't matter if the spell runs out.

The Goetia creature can henceforth manifest themselves in the aforementioned places when they please.

Meaning any before barring from Earth, would presumably be null at this point and they would be able to come back from that/manifest again: unless they're destroyed.