And why do we need to determine the max block size 20 years out? Indeed, it's impossible to make logical estimations about future transaction volume..... so why invent an arbitrary number and just go with it?
because you see now that it is nearly impossible to change bitcoin protocol. it gets harder and harder with more users. i am pretty sure if satoshi has forseen this discussions he would never introduced the temporary 1mb spam limit in the first place.
Block capacity is connected to transaction volume growth. So we have real world data to estimate our capacity needs for the foreseeable future. And those simply don't line up with 8MB or 8GB. Maybe in the future they will.... maybe. We can cross that bridge when we come to it. In the meantime, the emphasis should be on consensus. A conservative approach that scales to real-world needs rather than fantasized ones based on Moore's Law is much preferable in that regard.
Six years in, we have not hit capacity. Why are we planning on 8000x capacity 20 years from now? Just because? Incredible lack of foresight, and quite audacious to assume that we have every possible technical problem that may arise figured out, today.
we cant estimate transaction volume growth... one big war, one good media report, one country accepts bitcoin as main currency: one billion people trying to buy bitcoin in one week? no way to calculate something like with an algorithm.