Remember the fate of King Arthur in the myths. Mortally wounded, and then taken to Avalon to rest and heal, only to return if his nation once more needs him.

Considering Saber is Arthur, one could argue that this will also be her fate.

Putting aside the fact that as a Summon, she can never fully die. For as long as people consider her a hero, then she remains eligible to be summoned. No different from Archer being Shiro from a future timeline. He became the hero he wanted to be, and in exchange he became part of the "game". Never knowing rest, always fighting, seeing the innocent hurt, etc etc. Hence, why Archer is so bitter.