A minimizer will fill the lightcone to make sure there aren't paperclips elsewhere it can reach. What if other civs are hiding? What if there is undiscovered science which implies natural processes create paperclips somewhere? What if there are "Boltzmann paperclips"? Minimizing means minimizing!
My speculation here is that it might be that the "what ifs" you describe yield less positive utility than the negative utility due to the chance one of the AI's descendants starts producing paperclips because "the sign bit flips spontaneously". Of course the AI will safeguard itself against such events but there are probably physical limits to safety.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.