Date: 2018-02-16 14:25
With billion year lives, our future descendants might be altogether happy with a few hundred billion humans in total in the entire galaxy - especially if we find it has other inhabitants in it also. Say average of one or two humans per star, but of course concentrated in some places, or maybe traveling or exploring the galaxy? If so that might be why we haven't seen ETs yet, because though they are there and are maybe quite widespread through the galaxy, we are due a visit some time in the next million years or so, which to them seems a short time, just a thousandth of their lifespan perhaps, and nearest ones, maybe a small settlement a few tens of light years away. I think that's one possible scenario.
What I am pretty sure of is that any sensible ETI with foresight would not choose to fill the galaxy with unrestricted, evolving self replicating copies of themselves, disappearing beyond the light speed horizon to evolve or to create self replicating machines or modified lifeforms that could turn into almost anything imaginable or unimaginable. Before they do that, if they have any sense, they think through the long term consequences of their actions first, and find a way to do it safely for themselves and for the galaxy.
I think also that the best way to get to a galaxy spanning civilization in the Kardashev scale if we decide we want to, is to go through the earlier stages first. That you can't suddenly jump to a galaxy spanning civilization simply by scattershot sending colonies throughout the galaxy, self replicating robots, cyborgs, uplifted species etc. It might eventually use much of the energy available in the galaxy - but it wouldn't be a galactic civilization it would just be galactic chaos. I wouldn't call that anything like Kardashev level 8. You probably need something like the Star Trek warp drive, or immensely long lives at the least, or some other galactic unifying element, to have a galaxy spanning civilization.
So, first, before we even become solar system spanning in a true way, we need to be established in our own home planet, as the most hospitable place for humans. If we can do that, then we are established here for at least hundreds of millions of years into the future. If we can't, especially if we destroy our home world in some way or make it uninhabitable, I don't think we have a chance of getting any further, myself.
Then if we have the capability to make efficient use of our sun, that's stage 7 of the Kardashev scale , At that point you are not likely to disappear even when the sun goes red giant and then collapses to a white dwarf - if worst happens you just migrate to a nearby star with that level of technology.
Perhaps it's only when you are at that level and also developed either immensely long lives or warp drive or something of that order, that you are at the point where there is some possibility of going galaxy wide in a way that is safe for yourself and other beings in our galaxy.
There's no hurry here though. Perhaps at least for billions of years the most important thing is just to make sure Earth is safe. We can explore the galaxy with robots. Even without going as far as stage I of the Kardashev scale , with millions of years of future technology development, we'd be able to do almost anything if we wanted to. You don't have to use all the energy from the sun even that arrives on Earth just because you are capable of doing so.
I think there may well be ETs who have the capability even to go right up to the galaxy wide stage 8 civilization using most of the energy from all the suns in an entire galaxy if they so wish, but just choose not to, for various reasons, and stay at an earlier stage, and they might be the wisest and most farseeing of all the ETs. They might be galactic explorers - but see no reason to remodel the galaxy just because it is something they know how to do. And even with a population of many trillions, they could still have minimal impact on the vast galaxy. That might well be through choice, not because they are unable to do it.