My guess is that whatever an advanced civilization is looking for, they'll find it sooner than in 50 million years. Already the rate at which human technology is advancing is accelerating. The probes imagined in the study are only those that we can imagine given our own current technology. If one studies the problem intently, the only reason to send out probes early is the supposition that one will not learn much more later more easily. Thus, there were certain things we humans needed to know about the planets in our solar system to verify our speculation about how physical and environmental properties might be dispersed in a similar star system...and whether or not life could or could not be sustained under what conditions and for how long. Once we know all that stuff, technological advancement will either make the data irrelevant or moot...or so goes one line of speculation. Thus, early and limited probes might be sent out...but after technology develops beyond a certain point what good does probing do...probes and mapping of an entire galaxy pre-supposes that the intelligence behind such an effort would be interested in knowing that information...while it might seem fantastic and valuable to gather that info right now, in our own social and technological situation here on Terra, right now, it does not necessarily follow that we'll be thinking along those same lines at all in several hundred years at the current rate of technological progress at all. Perhaps our technologies will make having to "expand to the stars" seem like childs' play or seem irrelevant? In that case, there might be some distant futurial individuals or groups that really, really felt the need to know all there was about every last corner of the galaxy...but would that be seen as possibly a waste of time by the movers and shakers in that same society many centuries hence?