What would happen if all Bitcoin is mined and all Bitcoin is lost?
According to an article by Blockchain Council, all 21 million of Bitcoins will be mined by the year 2140. almost 120 years from now. Bitcoin supply will be completed and then miners will only earn money by confirming transactions. and earn fee from the fees.
This article is very interesting you can read it here:
https://www.blockchain-council.org/cryptocurrency/how-many-bitcoins-are-leftNow for your second question. I don't think there is any point in thinking about that. because that's not realistic. I don't think all bitcoins will be lost. there will always be Bitcoin in circulation. it will just get harder to get it because it's price may increase so much with time.
Even if we assume that all Bitcoin was lost and only a fraction remains, let's say 1 Bitcoin, then since it is rare, then its price will be astronomical and we will talk about fractions of it to be worthy, maybe there will be micro-satoshi or something like that.
The worst scenario is to have a hard fork and to get a recovery from months or years before the losses so there will be a possibility of retrieval.
Anyway this is very hypothetical and it will not happen in the current century at least.