I don't understand enough about that. You can also replace PoS with a protocol that has yet to be found.
In 20 years Biotcoin must be worth several million to be as safe as it is today. Or the fees or the transactions have to increase significantly. We know where the reward will be in 20 years' time. But we don't know how many transactions there will be then. I would guess not dramatically more than today.
I find this issue to be more about the economic aspect, and less about the technical aspect. Most consensus mechanisms that crypto runs on are actually based on the very same game theory aspect; giving up one thing in return of another. Replacing Bitcoin with another mechanism is unlikely to solve the issue.
In a similar vein, Bitcoin's deflationary design is intentional, and recycling old coins is not going to solve the issue. Changing the design entirely, either by unlimiting the supply or stealing some coins will violate the entire philosophy of Bitcoin. These kinds of ideas are best left to an alternative crypto, aka. altcoin.
Perhaps thermodynamic protection would be a better phrase. I don't know. Be that as it may SHA256 forces the use of energy that can hardly be reduced. Correct? I don't know how I could calculate SHA256 with significantly less electrical energy than Bitcoin computing systems do today. I think it is very secure today.
Calculating SHA256 is extremely trivial, in fact it takes nanoseconds for miners today to calculate a single hash (actually even faster). if you're talking about the energy consumption of the speeds at which miners are calculating hashes, then yes it probably cannot be reduced significantly.
But if there are fewer and fewer rewards, the price only increases by a few percent a year and not much more comes in via transaction fees, then this may change over time. Or am I making a logical mistake?
That's correct, because difficulty, or the amount of "work" is also a function of the revenue gained from the margin. I'm more inclined to think that this isn't as big of an issue as it seems because Bitcoin's network is generally secure enough even if some miners were to find it unprofitable to mine in the future. Prices are likely to rise for more than a few percent YoY, because deflationary forces are at play assuming a more limited supply. Revenue from TX fees can increase in the future if we can increase network throughput in a way or another.
In comparison with changing the entire consensus mechanism of Bitcoin, I'm inclined to think that these potential improvements over time makes the issue less significant.
Of course you can say, what do I care what happens in 15, 20 or more years' time? I personally like to think about things like this from time to time. But I understand that you are not interested in the subject. There are no problems at the moment.
Generally, I think its a worthwhile discussion if we're able to find realistic proposals that can potentially replace PoW and PoS in the consensus scheme. These are topics that are discussed previously and the outcome is usually that they aren't all that good and it wouldn't make sense for Bitcoin to switch. In fact, as it stands, we don't have a comparable consensus mechanism and it would likely face too much complexity to attempt a switch, for an issue that likely doesn't exist.