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.