I propose the following rule to determine the block size limit, once the block reward is low
The block size limit would increase (or decrease), by X%, if total transaction fees in the last N blocks is Y Bitcoin or more (or less).
......
I am aware miners could also manipulate fees by including transactions with large fees and not broadcasting these to the network. However why would miners in this scenario want to manipulate the limit upwards?
The fear is that a cartel of big, centralized, have-huge-data-pipes miners would drive out smaller miners by forcing up the block size high enough so the smaller miners have to drop out.
Elements to make mining competitive are cheap power, good connectivity, cheap heat management, and technology development. Cartels can form that take advantage of either of these elements. As long as all of these elements are not overly abundant in only a few geographical regions, mining can stay decentralized.