It is low.
I have followed a number of "game as a service" games and every single of them realised that it is always better to upgrade the existing client and rewrite it module by module (effectively making a new client, like Argo's Ship).
This is what FGO doing right now, making a number of QoL updates and full campaign to attract new players (pure prisms, better story clear rewards, etc.).
And honestly new game would suck, as it would have maybe 30-50 Servants on release and you'll have to wait for your favourite ones to be added (if ever).