Pitterpatter please do not spam. (i hate spams worse than double posts)
We can't simply just talk about the ending, what's more important is the storyline. As you say 'saphira definitely will get a mate' well we don't know about that,, what if all the male dragons somehow die ? And about galbatorix, what if afterdeath he'll become a wandering spirit waiting for re-incarnation or something so then Eragon need to chase him around the world ? (uhm.. it does sound stupid)
But yeah,, following our 'novice logic', the idea of Eragon leaving Alagaesia is absurd and looks like an action someone will do when he's cornered and has no other option.
We should think like this (perhaps):
Why Eragon leave >>> because he need to >>> because he's doing something important >>> something NOONE else besides him can do it >>> what makes Eragon special? (so he's the only one who could do it?) >>> there are several possibilities. First is the fact that he is a rider. Second it's because of Saphira. Third is still-the-unknown cause.
OR
Why Eragon leave >>> because he need to >>> because he's running away >>> there are several possibilities. First is his life and Saphira's are in danger. Second is
someone else's life is in danger IF Eragon stays in Alagaesia (it could be a curse or a bad destiny, or any badluck cause by magic)
OR
Why Eragon leave >>> because his people don't want him anymore >>> because he's turned bad and evil !!
OR
Why Eragon leave >>> because Alagaesia, along with his finest memories, doesn't exist no more >>> because the ultimate battle destroyed everything, turned the forest into grassland and turned grassland into desert,, there's no survival left !! (hahaha,, reminds me of Dragon Ball's battles

) but this is very unlikely.
We're talking about "leaving
forever",, not a mission that last for 10-20 years... but for eternity!! As you see... there's so much possibilities... So many alternatives endings..!!