Bitcoin will stay forever as long as someone has a copy of the blockchain (around 300GB now). The cost of which is negligible today.

Mining is only necessary for making transactions, and it doesn't have to be expensive. It only is because of the competition. That expensive competition is important for stability but in theory, the entire system could run on a smartphone.

And it could run without newcomers. If everyone in the world had bitcoins, making it impossible to have newcomers, it would still have value, in fact a lot more than it has now.

Bitcoin may be a bubble, but it is not a Ponzi scheme.

Gold has some value as a technical material and because of its (subjective) beauty, so it will never crash down to zero, but it could lose a lot of its value if we decided that it is not worth hoarding.