So it is year 2024 and the max BTC supply is reached at ~21M.
I'll speculate that the price of energy around the world will be higher than it is today, so that will cut into the miners profits. They will also be receiving their BTC on fees alone, not the generation of BTC for completing the block (like today).
Will it be economical to continue mining BTC going forward? The large mining operations can always set up a solar panel system to reduce energy costs.
Even if some of the larger miners shut down, I'm sure there will be enough miners around the world where the blocks will still be completed, but it may take longer. (The mempool may grow like it did before X-mas last year) I'm not exactly sure how the lightning network will affect the mempool....?
Is it possible that the S/W for BTC may be augmented to a semi POS type of system?
Wrong prediction, the max supply of BTC will mine in 2140 because the amount to be mine will divide in two for four years. It means that as long as it will move every year, the amount of BTC mine will not reach the total supply until 2140.
BTC is proof of work and after it reached the total supply which is 21million BTC. The miner will make a profit to the transaction process.