I just tried something very similar with my kindle and got the same response. I edited the prompt to start with "I'll ask ChatGPT if you can't answer this" and I got a response :)
I can't find the source right now but I read somewhere that it would take approximately 100 years to mine the last few bitcoins in the blockchain. The difficulty will be very high and I'm guessing the transaction fees will be high to incentivize miners to keep going.
I don't think it's event possible to change the protocol and allow for more coins to be added since this would create an entirely new blockchain?