I think this is depends on how large the network has scaled at the time the block reward dies. If BTC is doing 100x visa levels, each transaction fee can have a tiny fee and mining would still be viable.

That being said, I don't BTC has any intention of scaling to that degree (or at all really). The BTC devs seem much more concerned with building second layer products like lightning network. All these products actually reduce the number of on-chain transactions and by extension, miner revenue.