Illya suffering is a multiverse constantSeeing as whatever world Illya now resides in has a functional Gaia, some of the subtext seems to imply that in order to be able make use of Gaia's assistance, Illya has essentially become the Brain of the Planet in this worldline. Something which can probably be accepted because apparently there aren't any other living beings left. Therefore it has become Illya's role to observe and remember all that happens to this world.....forever.
Also explains how she casually builds and destroys the house.