You also forgot to mention that at some point, when the 21M coins have been mined, the inflation rate will become 0%, or even a minus, as some people lose the access to their coins, and new coins are not being produced, that makes us at a decrease in the amount of access able bitcoins.
It is going to be hard for miners to keep mining when the block reward is basically zero, as fees will be the only pay they get.
I don't think 20 years ahead, because that is rarely sensible these days. Not sure whether bitcoin will still be the leading non-governmental currency in 20 years.
But before that, it could be interesting to see the miner rewards in dollar or gold terms. Let us not forget that, while the reward in bitcoin sinks, the value of bitcoin rises. Miners have perhaps roughly constant rewards in terms of purchasing power, or even growing rewards.