I think perhaps you're  underestimating the degree to which utility can be added to the main chain to encourage fees.

For example, lightning channel open and close transactions are more valuable, via aggregation, than simple money transfers.

The value of higher utility transactions goes up fairly quickly.

There is no end of the possibility of fee levels in response to increased utility

Other networks have clearly proven the extremes of this, with rampant fees appearing rapidly in response to higher utility levels

Because this has already been proven on other networks, we can plan to gradually  increase the utility of on-chain transactions in response to reward reductions

This should be more than sufficient to offset and maintain sufficient security 

... Hence the title of this thread.