The security of the network depends on the difficulty. It’s true that in the absence of bad actors the network will run as long as fees are nonzero, but the whole point of bitcoin is that it’s supposed to be resilient to attacks.
If usage is high - fees will be high, and the cost of an attack will be high.
Security scales with usage, as does the incentive to attack. As long as usage trends in the same direction as value, I don't see the problem.