What would happen if all Bitcoin is mined and all Bitcoin is lost?
Lost means that if it can't recover then it will definitely become worthless. And if Bitcoin starts to constantly disappear from the total supply then of course Bitcoin will slowly increase in demand and the price will likely increase a lot.
However, since Bitcoin is decentralized and there is no way to increase the supply like paper money, the more Bitcoins are lost from the supply, the more pressure there is on the amount of Bitcoins left.
You don't understand, the OP means if all the 21 million Bitcoin are mined and by God knows what unfortunate event all got lost (the 21 million Bitcoin got lost), what would happen. From my perspective I'll say we forget that Bitcoin ever existed and we go on with our life's but that's so unlikely to happen because Bitcoin isn't centralized just as you have highlighted and we can't lose all the Bitcoin all at the same time unless something was to happen that all the 21 million Bitcoin that was created get destroys through some kill switch and we don't have anything called Bitcoin anymore but that too is unlikely to happen.
Nobody controls Bitcoin so they can't just make all the Bitcoin to go missing within trace.Bitcoin was the blueprint of the industry and without it there will be no cryptocurrency industry, if satoshi was alive and all the Bitcoin was to go out of existence, he'll have something to do about the situation to win back the trust of the community and they'll back him.
Maybe a fork could happen and we get new Bitcoin with the same features and values and it's left for the communication to decide if they'll still trust satoshi or not as for anything to be able to remove all the current Bitcoin form existence, then it has to be beyond human and normal computer capacity.