What happens to Bitcoin mining profitability once most BTC is mined?

I've been thinking about the long-term future of Bitcoin mining and had a few questions for the community:

• When most Bitcoin has been mined and block rewards diminish, what happens to mining profitability? Will it become unsustainable?
• Is it possible major exchanges like Coinbase and Binance will eventually start operating their own mining farms to secure the network and business?
• With electricity costs potentially dropping due to advancements in solar and modular nuclear plants, could cheaper energy solve the profitability problem before it becomes critical?

Would love to hear thoughts on how this might play out over the next decade or two!