Well...that's the thing, really. Servants are some next-level shit, to put it bluntly. Without an ongoing Holy Grail War, it's impossible for even an above-average mage to have the energy reserves to sustain one's existence. Even if they survive and win outright, once the war's over? That's it.

[His voice very carefully measured and controlled in the picture of steady calm, Waver snapped his fingers to underline the point.]

Either they die in combat, or they disappear once their wish and that of their Master is fulfilled. [...ostensibly, but you know, grail shit.] Vanishing back to myth and legend.