So it is year 2024 and the max BTC supply is reached at ~21M.
Wrong. We will be there approximately at the year 2140. The bitcoin as block reward or
subsidy halves every 4 years that's way it will take much longer than you think.
Will it be economical to continue mining BTC going forward?
Hard to say but someone should've discovered something that would make the production of electricity cheaper. And also someone should've made something significant in making mining more efficient.
Is it possible that the S/W for BTC may be augmented to a semi POS type of system?
Not sure how to answer this but I doubt this will happen.
This may sound foolish but given difficulty can be scaled upwards can it not also be scaled downwards, if we reached such a scenario where mining was inefficient could the difficulty not just be reduced so that it is then more efficient?