Bitcoin can't always be proof-of-work. Given the capped supply, the mining rewards will eventually diminish to the point that the network is no longer secure. It will have to transition to proof-of-stake or some other mechanism long before that happens.
It intends to rely on fees, i.e. to have a constant backlog of high fee paying transactions. We'll see how that plays out within a few decades as the block subsidy becomes insignificant.
Transaction volume goes up and down. Just look at ETH gas prices - varying between 1 and 1000 month-to-month. BTC holders aren't going to be OK with a temporary lull in transactions leading to a double-spend attack etc.
