What happens if the global internet infrastructure deteriorates in bandwidth or connectivity over that timeframe? What happens if the global electricity infrastructure deteriorates so much that mining by PoW will not be possible any more ?
I would say that if global internet infrastructure deteriorates so much, that we probably have MUCH MUCH bigger problems to handle than caring about a crypto currency on the internet. Maybe, finding a gun and defend ourselves, and trying to find food or something. Mad Max style things.
Crypto currencies are a side product of high network connectivity, cheap computing and sufficient electricity.
The problem with block chain based crypto currencies is that they start from some most probably erroneous principles. Full trustlessness is very, very hard to implement, and the price to pay is way, way too high for a real-world situation where partial trust is often a good investment. Block chain based crypto currencies, writing all transactions on a global ledger, are extremely wasteful on computing, storing, and network resources ; but as these resources grow over time, and as adoption of these crypto currencies is NOT THAT IMPORTANT, there is a natural throttling of the adoption by the technological cost. What is strange, is to limit this resource artificially with block limits, instead of having the resource price be a natural limit.
Block chain based crypto currencies will never equal fiat style payment systems, because the investment in trust in these fiat payment systems allows for HUGE advantages in terms of capacity. The price that block chain based crypto currencies pay, is that they try to adhere to full trustlessness, which is too high a price to pay to compete with trusted systems.
This is like driving to your work in a fully blinded tank, with all reserve food to be able to sustain all kinds of attacks: for sure, your system is more "secure", but the resources wasted on that are so big, that you are not competitive as compared to the guy taking his bike. Your costs in terms of maintenance, consumption, capital cost etc for your tank are so big, that you waste all the money you win in going to work in the first place. The guy on the bike takes the risk of being shot down, true, but at least, he's way, way more competitive in earning money than you are.
Block chain based crypto currencies are over-protected by trustlessness, and hence, are not competitive with fiat systems.
Of course, you can transform block chain based crypto into less trustless "banking systems" on top of them. That is more or less what the LN tries to do. You will get a network of LN banks (big hubs where you have "an account", that is to say, a small-user payment channel).
Yes, you can build faster and lighter structures on top of bitcoin. By giving up trustlessness. And building a fiat system.
My idea is that the only use of crypto currencies are those niches where fiat doesn't work. Those niches are not that important, and advances in technology will push the limits of what crypto currencies can handle to a level where most of those niches are addressed. In other words, the relative scarcity of adoption that crypto currencies allow will be taken by the most useful niches that need it.
And all the rest will be done by fiat. I don't see the use of building another fiat system on top of bitcoin: we already have that. Let crypto currencies exist for those niches where you can't do it with fiat.
And then, one day we will invent better systems than block chains. That day may not be far away.