What happens if bitcoin mining stops or becomes too expensive? Are there mitigating factors, probably the adoption of renewable energy?
It is very simple, if Bitcoin mining stops, as in no one is mining anymore, the network will automatically come to halt. There will be no one to solve the block, thus there will be no confirmation of the transaction. With that, all the sent transactions will be in pending mode, and if they persist the network will die.
If the mining, on the other hand, becomes too expensive, many miners would probably shut down and the strength of the hashes may trim down, but do not worry, the mining difficulty of Bitcoin adjusts to the strength of its hashes, so the blockchain network to confirm the transaction will still continue.
Is that within the realm of possibility?
Yes, it is in the realm of the possibility, albeit being slim, since many mining farms are resorting to renewable energy.