The argument here is that working now is worth it because in return we'll earn utopia. But that argument will equally well count for fighting alien u/FAI and entropy itself.

Not equally well. The tiny period of time that is the coming century is what determines the availability of huge amounts of resources and time in which to use them. When existential risks are far less (by a whole bunch of orders of magnitude) then the ideal way to use resources will be quite different.