Yes Bitcoins 21mil cap can be removed. In a hard fork, we would essentially have Bitcoin and another coin branch off. Bitcoin will continue with the 21 mil cap but the new coin will have a new cap or no cap. I am sure there must have been a fork in the past where this was done. One coin will continue being used while the other dies off.
It’s a good question to think about actually. What happens when the 21 mil cap is reached and miners get no more block reward. What will incentivise the miners to continue mining. Will transactions fees have to compensate for the loss of block reward? In that case wouldn’t tx fees be incredibly high? So where is the utility as a currency at that point? I don’t know the answers to these. They should be another thread all together to be honest.
Exactly the only way is to create a new cryptocurrency in the form of a hard fork and put up a higher number of supply but for bitcoin it can’t just be done the way we see it, Satoshi doesn’t have that power again to make an individual decision even though he is around that’s how decentralized bitcoin has grown to. A consensus must be reached before any effective change takes place if not the change wouldn’t be effective and it will just result into creation of a new coin just like the likes of Bitcoin Cash, Bitcoin Gold and Bitcoin Diamond and no real use of it. The issue do this 21 million bitcoin is what even adds value to bitcoin because something that’s scarce is also expensive.
Yes after all bitcoin have mined, Miners will definitely depend on transaction fees as the only incentives, we have discussed that many often in this forum. Miners already are having incentives from transaction fee which are much more than the block reward itself. A thing you need to know is transaction fee are charged in bitcoin, base on current rate of bitcoin price raise the price increase might be actually help, for example after the second bitcoin halving when the reward was reduced to 12.5 bitcoin you can clearly see that the current 6.25 is almost equivalent to it in terms of the fees. So this could actually help in the future too but the problem will be, will bitcoin be ok for micro transactions due to the fees that will be paid then?