>>725258134
Nothing because the Holy Grail doesn't actually do that
The Greater Grail is just a massive natural wellspring of mana that the three families primed 200 years ago for the specific purpose of completing the Holy Grail War ritual, which is to utilize that gigantic mana pool at the exact moment the summoned Servants' souls return to the Throne to blow open a path to The Root and reclaim the Third Magic (or some other kind if you're not an Einzbern)
The Greater Grail is basically a "wish-granting machine" able to direct that vast wellspring of mana toward any particular desire, but it has to be something realistically attainable with magecraft and it doesn't work anymore because the Grail has already accepted another wish

There are other Holy Grails in the series but none of them could accomplish this either, time travel and other violations of causality are Magic