Or when, a few decades from now, the block subsidy has became insignificant after repeated halvings. If you then having a period without lots of high fee paying

Or when, a few decades from now, the block subsidy has became insignificant after repeated halvings. If you then having a period without lots of high fee paying transactions, Bitcoin could fall victim to a 51% attack double-spending huge amounts. The immutability of the 21M limit that is Bitcoin's essence could also prove its long-term downfall.