If you interact with the Endless that give sidequests after the terminals are off, they generally just accept without much issue that they were turned off and in one case even say that keeping them "alive" at the cost of the lives of others would be unconscionable.

Quote Originally Posted by Kirutsuki View Post
Let me ask you this, if a solution is found later. Someone just comes in and asks "why didn't you do this or that?" and it would have kept all in Living Memory alive and not force Sphene to invade other Reflections.
If we're talking about hypotheticals like these, then they can simply be turned back on. We know that various Endless have been turned off before; particular people were turned on and off multiple times over the years as the aether reserves that sustained them dwindled. If their existence was genuinely threatened by being turned off then they were already on the verge of nonexistence.