“How long can any civilization last before it implodes in some way? Maybe a couple thousand years??”
By human standards, going by all of human recorded history? There is just no way of knowing, we don’t have enough data on how “alien” actual aliens could be to us. If there are really old alien civilizations, they had to start at some time. Maybe they could really different than us, not just “men” in a different shape, but having completely different goals and drives. Or maybe they live a couple of thousand years or longer.
It is interesting to speculate why there hasn’t been any contact yet. You would think that anyone advanced enough to reach the stars in a reasonable amount of time would do so, and be powerful enough to find other sentient life easily. But maybe that’s just our human penchant for exploration. Maybe there is something out there that squashes the more bold races when they make themselves known. Maybe most are species that just aren’t interested in exploration, alien contact, or even knowing anything we could relate to.
Freegards
Or maybe there aren’t any alien races.