With current technology, the main problem would be the latency. To the moon its measured in seconds (not milliseconds), to Mars in minutes, half an hour to an hour. Syncing the blockchain on a solar system wide level would be very difficult, not impossible, but tricky. You could easily have two blockchains running in parallel on Mars and Earth at the same time for long enough periods to do harm. This is definitely something that was not in Satoshi's mind.

One promising future technology that could fix this, is communications based in the properties of quantum entanglement. So yes, not all "quantum" stuff is evil, quite the opposite in fact.

The reasoning about a catastrophic event is unnecessary, if such thing occurs there would be far more pressing issues than keeping the Bitcoin network alive. No, but if in a few centuries later humanity starts populating the system, it becomes important. For interstellar distances you can pretty much forget it, you would be fighting against time itself... Unidirectional communication could be theoretically possible thought.

I can imagine a future where some colonies are setup both in space, such as artificial places at lagrange points, and in some suitable places like Jupiter moons, Mars, etc. As i said earlier, without quantum entanglement communications take minutes to hours to reach points. Even communication from earth to earth using satellites is laggy due to the distance it takes to reach orbit and back, or if needs to be relayed further. This is a physical issue, nothing can be done to lower the latency (short of quantum entanglement), bandwidth/bitrates can be improved, but not latency the speed of light (assumed constant) is in the way...

Did you know? Light from the Sun takes 8 minutes to reach Earth? Yes light, just a different wavelength... But the Sun is in the center, what you do when Earth is opposite to Mars? You already have those 8 minutes plus the longer distance towards Mars... And when the astrophysics start measuring in LightYears, better get ready because that's actual Earth years that light takes from point a to point b... Before taking in consideration time relativity.

Don't worry, if we don't go extinct, our descendants will have fun figuring clever ways to overcome this. Most fiction and even some science-fiction tends to ignore this. Star Trek instant bidirectional communications with Earth? They better have full dominance of quantum entanglement, else...

let's not put the cart before the horse. cryptocurrency needs to actually go mainstream first---and not just as a speculative investment to be hoarded. like hatshepsut93 points out, bitcoin and lightning can't begin to handle all the world's transactions. neither can other blockchains, which are either less scalable or will suffer from the same inefficiencies as bitcoin.

everybody should just get used to cryptocurrencies coexisting with fiat, which isn't going anywhere as long as nation states still exist. i sure hope physical cash isn't going anywhere either.

It is still needed to be fair. Crypto can potentially usurp the role of fiat in the future, but that is still very far. Crypto at the moment is just at its infancy. Besides, government and crypto still dont meet eye to eye at the moment so removal of fiat currencies would cause major mayhem across the globe.
Plus, letting crypto take the role of fiat with how most people are still ignorant of its existence is pretty much impossible. Influence of crypto should slowly spread over the globe and only then can we start the improvementsbin terms of its role as a virtual currency.
As you said, issues still plague the usage of crypto as a fiat currency but that should be patched up in the future with how technology keeps on evolving.
Long story short, I believe it can, but not now. So at the moment, both should co-exist and let each other prosper.

You are mixing terms "cash" and "fiat" in many places in your posts, it makes it really hard to properly answer your question. Cash means physical money, fiat means government-issued money not backed by anything.

Should cash exist? Yes, because electronic payment methods rely on the Internet, electricity and special devices, and those things can't be always available, especially in remote places.

Should fiat exist? Crypto is technologically incapable of handling all fiat's transactions in the world right now, even with Lightning Network. So, it's kinda pointless to discuss whether crypto should replace fiat, until it's actually possible to do so.

OP, I like the idealism, but some of us have come here for the money, but stayed for the revolution. Understand the incentives, encourage the greed, then it will lock everyone in.

We still far from this scenario, but at least with the Germany adopting the block chain technology is one step ahead towards satoshi's vision. We still don't see how the other bank sector will adopt block chain today. but if we see more countries are following the footstep of Germany, the bank sector in the world will leave no choice regarding this matter and that's when the time we will see a massive impact on bitcoin price.  maybe some other countries at that time will also make bitcoin as one of their national currency and so the other altcoins as well.

I don't think so.
India has higher unemployment rate than Russia and also much larger population.
The unemployment rate in India is 6% and in Russia is 4.3% [Ref.]. Also, the population of India is 10 times the population of Russia [Ref.]. But we don't see India in third top visitors of freebitco.