Today, each block has an average of around 4000 transactions. If today reward of 3.12 Bitcoin would go to zero, that would mean that each transaction fee, to compensate, would be 80000 sats, around $75!
Assuming that in 2140, all the world wealth will be transfered to Bitcoin, and assuming that each Bitcoin would be valued in todays dollars at $23M, that means that each transaction of 80000 sats would be worth around $18k!!!!