A lack of high fee paying transactions once the block subsidy becomes insignificant in a decade or two, would make bitcoin susceptible to 51% attacks.
I think by that time, if the network effect has grown further and more parties have invested large money in it - they’ll just find a way to fix that.
For example, they may move to proof of stake.
It would ruin the decentralisation imo, but it doesn’t matter because it would save their value (money) and that’s what every participant will want first and foremost
It goes to show that because it’s software that holds money, when push comes to shove, people will just agree on a new version to save their money