I think 120 years is enough time to find a solution to keep
BTC alive even when all coins are mined
But that is the future which most of us will certainly not be able to see and therefore is quite irrelevant what will happen then.You should focus more on the next 10-15 years because somewhere around year 2032 almost 99% of total
BTC supply will be mined-and that 1% will remain for the next 100+ years.Therefore, this "problem" is actually much closer than most thinks,but also this means that at that time(2030) there will be very small amount of fresh mined coins on market,so if demand be significant-price will follow that for sure.