When it reaches its supply limit, miners will stop receiving block rewards for mining transaction blocks which is their main incentive for mining and keeping the validity of the blockchain.
The main incentive is currently the block reward. Over time it'll stop being the main incentive. I suspect they'll earn much more in fees than now which is ~0.1 BTC.
If we want Bitcoin to become a global currency that is used for primarily every transaction, having fees is an easy way to deter new people from adopting Bitcoin as their main currency.
The speed of a confirmation may also discourage them from using it. That's why we have the Lightning Network. To make everyday micro-payments instantly and with a nearly zero fee.
I think instinctively, adopters of Bitcoin will immediately turn to raising Bitcoins supply cap (probably by 2x) to solve this issue
This will never happen but answer me this: Why would
this solve the issue? Won't they also need more incentive after it reaches 42 million? Sounds like a paper over the cracks.
One solution to this problem will be to make the amount of power used to mine blocks significantly lower as that will incentivize miners to mine blocks with lowers transaction fees.
The amount of power is calculated by the difficulty. The difficulty is calculated by the frequency of the block generation. If it becomes more frequent than 10 minutes, difficulty increases. The opposite happens correspondingly. This frequency is determined by the law of demand and supply. If the equilibrium market value of bitcoin increases then the miners have a greater incentive to use more power, because they earn more money.
You can't determine the power required, that's a result of free market.
I would love to hear you guys opinion about this issue because we should be prepared for when this happens.
I'd recommend you to start making questions regarding this as you have definitely misunderstood tons of things.