> So it might end one day.

Only if the mining stops abruptly. Otherwise, the automatic difficulty adjustment allows it to keep working even with a reduced energy expenditure. In the extreme, a single computer running in a museum could be enough to keep the blockchain working forever.

A single computer in a museum would be vulnerable to 51% attacks. It would also be too illiquid to recoup costs.

But the 51% attack would cost more than the value of the Bitcoin you could get by doing it, which is presumably ~0 since the only node is a museum computer.

The difficulty is adjusted automatically depending on how many miners there are, so it can go down.

That might be one of those things that works well in the short term but may also have severe tail risks.

I heard a similar story about an algorithmic stablecoin called Luna (I think). The analogue of difficulty adjustments there worked fine until one day it didn't.

They can ultimately change the code if needed. Luna also tried that IIRC.

Indeed, one can "change the code" to do anything... including things like increasing the bitcoin supply arbitrarily. Bitcoin's scarcity is a social construct that will be changed when the people who control the nodes want to change it.

Luna was a very different situation, it was nothing to do with difficulty adjustments.