Suppose that there is no need to have in the equation living beings. So you have your von Neumann probes assimilating all the matter in their path into new von

Suppose that there is no need to have in the equation living beings. So you have your von Neumann probes assimilating all the matter in their path into new von Neumann probes, lets call them Borg von Neumann. And they are nodes of a collective /distributed "computing" scheme called Alien Civilization 3.0. Still you need time to stop, make your conversion, pick next target, accelerate and deaccelerate. And you may be turning everything into nodes, i.e. suns, exoplanets, etc. As we find stars pretty far away and exoplanets in far away solar systems, we are safe for some million years at the very least, even with relativistic speeds. And still, it is about what we think with the wrong mindset and a lot of unknown unknowns to properly understand the problem. Expanding may be obviously wrong for advanced enough... something.