where's the profit motive in building a fleet of von Neumann probes to 'eat' every solar system in the galaxy? Who is going to pay for this effort, and what returns do they expect?
You only have to build one Von Neumann probe, and there's a massive benefit to taking over your light cone: You make sure nobody else does.
Even if million-year life spans are out of our reach, why does one have to live to see the full fruits of one's effort? The world doesn't cease to exist when you die.
So this hypothetical civilization has conquered aging, death, and the drive for rapid gratification, but fails at elementary game theory?
Even if there were multiple civilizations in a galaxy, a single "defector" civilization would have vastly more matter under its control than civilizations that did not use Von Neumann probes to expand. With such a huge advantage, they would quickly destroy all competitors.
Sorry, but I'm still not getting the 'advantage'. This strikes me as one of those games where the only winning move is not to play.
Fortunately, the loss of its empire is a process that always seems to leave the nation in question better off, at least in modern times. As I see it the only possible justification for total galactic conquest is self-defense: do unto others before they can do unto you, more or less as you put it. Any advanced civilization that doesn't see that as a self-defeating justification is so different from my way of thinking that I couldn't possibly deal with them on any terms, offensive or otherwise.
Basically, what you're saying is that the only way to survive and prosper in the Universe is to be a murderous, psychopathic douchebag on a galactic scale. Not down with that, sorry.
Were these your words, or not?
You're projecting human flaws and attributes on an advanced race that would, by necessity, be almost nothing like humans. It simply does not follow that because the solar system has not (yet) been turned into grey goo, we must be alone in the galaxy.
The push for the moon was extraordinarily costly - but an entire nation, the richest on earth at the time, was behind it. It was a national effort, a calling together of an entire people, a lining up of all effort and a relative lack of detractors.
There are plenty of prior examples of this type of cohesive effort - Britain in World War 2 springs to mind. But many others in prior history as well.
Therefore I would say the lack of decade-spanning effort is not because people now are different, there just is a paucity of generational ideas that get everyone excited. If something falls down on a partisan basis, then it's just not good enough as an idea to unite an entire people. History, after all, is really a long string of dull years punctuated by exciting events.
The LHC may be the last of it's type, but then how many people are really excited about particle physics, no matter how good the possible benefits might be? I would not say our attention span is narrowing, merely reverting to the mean.
That's quite a leap you're making. One very strong counterargument boils down to simple economics: where's the profit motive in building a fleet of von Neumann probes to 'eat' every solar system in the galaxy? Who is going to pay for this effort, and what returns do they expect?
If it takes a million years to colonize a galaxy, then unless you plan to live for two million years you aren't going to see the full fruits of your effort.
As a race our attention span seems to be narrowing. As far as I can tell, despite or perhaps because of recent advances in applied science, we're actually losing our propensity to engage in multidecade R&D projects. The next LHC-scale project is going to be an almost impossible thing to sell to the governments that will have to agree to finance it, and it would take years for us to return to the Moon if our survival depended on it. We are most definitely not moving in a direction that will lead to the sort of expansionism you're talking about.
Edit: here's another thing, with regard to the absence of 'radio bubbles' from other civilizations. It's really, really dumb, from a technical perspective, to transmit RF signals that are distinguishable from background noise. It means you're wasting power and throwing away channel capacity.
Look at an HDTV transmitter on a classical spectrum analyzer sometime, and you won't see much in the way of coherent structure -- you'll just see a pedestal where the noise floor seems a bit higher than usual. This means that the 'radio bubble' is not a bubble, but two nested spheres with only about 100 light years of space between them. To observe emissions from an advanced civilization, we need to look at just the right time, between the development of RF technology and information theory. Otherwise we won't hear a thing.