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.
Even though the difficulty might increase, isn't it possible that (even with the electricity thing) that the gpus that far in the future will be super powerful? What if they start mining coins without any issues at all? Does that change anything about how long it'll take to get to 21M or am I completely wrong?
I don't know much about the workings behind mining and I've always wanted to know. I'm sure these are questions that others might have.