Well, since the problem of even figuring out if you are alone is nontrivial why would you opt to do such a astronomical operation because of a hypothetical? We most certainly would not.
Why the pessimism? We do know that we currently do not have any plans for massive expansion through the whole Milky Way, and even if we made such plans, we would definitely include safe zones and ample space for other species we might encounter. If we are that good, maybe other species are no less good? That's not an unreasonable assumption, though ultimately anthropocentric, of course.
But the point I was trying to make was different. I should have said "There is not enough reason". There are just as many hypothetical reasons for as there are against. We really don't know.
>expansionist pressure seems to be the safe assumption.
Then again, the universe is very, very large...
We haven't really done such a great job on or own planet. What makes you think would do any better in the stars?
There are definitely reasons to believe that.
You certainly don't want to be in a galaxy where someone else does it first: if you can't coëxist with them, you're probably going to be wiped out.
Large-scale projects like Dyson spheres would, I imagine, require vast resources taken from multiple solar systems. So even if you're not colonizing every star, you're probably mining from many of them.
Anyway, just a couple of reasons that spring to mind. Given our data set is exactly one intelligent civilization it's hard to draw conclusions, but expansionist pressure seems to be the safe assumption.