The idea is that bitcoin is widely used with millions of transactions on each block, so those fees will be enough.
I don't see that happening. It's close to impossible to happen actually, especially if you look back at how we're working on ways to scale off-chain rather than on-chain.
Interacting with sidechains or Lightning requires on-chain transactions though. So we can't have exponential growth of throughput off-chain without it heavily affecting on-chain fee dynamics. Either the block size will be increased or fees will significantly rise. I think it'll be some from Column A and some from Column B.
In the long run, miners will also be scooping up the fees generated through Bitcoin's side chains that they merge mine, so if the activity on these side chains explodes due to all the garbage that runs on top of them, miners will automatically benefit from that. In other words, there is no real need to worry about miners or the security of the network.
Is that the consensus opinion? I don't really keep up with this stuff, but I thought there were a lot of security issues with merge mined sidechains.