Indeed, I've never understood the "limited" argument with gold. When we become a galactic-faring species, there will be such an abundance of gold in asteroids that it will become more common than water. Bitcoin on the other hand has a hard cap of 21 million units. No matter how great and intelligent we become as a species, not everyone will have access to an entire bitcoin.
There are a few flaws with your argument. Gold will never be more common than water. The abundance of water in the universe is much greater than gold. As the human race expands, we will always find way more water than gold. Furthermore, if you took all the asteroids in our solar system, the combined mass would be less than the Earth. It will probably become way more economical for us to start mining the sea bottom for gold than to mine asteroids.
Also, due to the fact that we can only communicate at the speed of light, this prevents the Bitcoin network from expanding much beyond Earth. Quantum entanglement communication devices cannot be made for the simple fact that as soon as you try to force a particle to certain state, it is no longer entangled with the other particle. If you set up a colony on the nearest star, it would take 4 years for your transaction to be communicated to the nodes back on Earth. The only way that you could get Bitcoin to reasonably work with such vast distances would be to run your own fork.
Some interesting points you raise. I don't doubt that whatever planets we colonize or mine in the future will have more gold than earth, so perhaps not asteroids but nevertheless there will be an abundance of gold in space with a technically unlimited maximum supply.
I'm of the opinion that in 5,000 years or so from now we'll find a way to do faster-than-light communication, considering our understanding in physics has only boomed over the last 200 or so years. Science has a habit of always thinking "now we get it" only for newer theories to come to light and replace older ones. Surely this will happen with quantum physics as well. In the event it doesn't, what's your opinion about using something such as a Tangle? Which could record and verify transactions off the main network and add them back to it later when they connect again?