[quote=@Bishop]
(The Command Spells's) ...their truest purpose is to allow the winner of the Grail to force the Servant to kill themselves so that the ritual to reach the Root with all seven Heroic Spirits can be completed.
[/quote]

[quote=@Animus]
[@Bishop]Yeah, Lancer in Fate Zero was forced to take his own life iirc.
[/quote]

So was Lancer in Unlimited Blade Works; you can order a Servant to do anything with a command spell but that's not the point of that quote.

From my limited understanding the way the grail war works is that by filling the grail with the mana of ALL SEVEN Servants the grail gains enough power to grant the winner of the war any wish they want. Every Servant needs to die for the ritual to be completed, not just the other six; in other words a Servant can never have their wish granted, or at least not and live to see it, since they need to die as well for the grail to be complete.

I could be wrong though and that could only apply when using the grail to reach the root. Maybe other wishes take less power and therefore you'd be able to have your wish granted even before the war ends if you can get your hands on the grail and enough Servants have died to fill it up. Plus there are examples like the Great Grail War in Apocrypha and this case where there are more than seven Servants so not every Servant would need to die, since it's to do with the amount of power in the grail and not a hard rule of all Servants need to die.

I don't know. Fate is weird.