What happens in 2140 when the last BTC was mined with the confirmations of the transactions in the blocks and thus with the block rewards. There should not be more BTC than Rewards, because 21 Mill BTC were gemined.
How are the transactions then confirmed?
Ye I can imagine it switching over to POS as an option but there is also an option to tip when a miner decides to mine a block. I can see this tipping implementation along with transaction fees as being sufficient enough to incentivise miners to secure the network