The ability to support more usage without exploding RC costs depends on technical limitations. How fast can nodes process blocks of a given size, how feasible is it for nodes to expand storage, how much memory is needed to run a node and how expensive are servers with the required memory, etc.

Witnesses can easily increase the block size, but that doesn't address the other resource constraints. The way the code works now, it would need to be updated (via soft fork) to increase RC resource pool sizes if increased resource use becomes more feasible. That could be due to a combination of improvements in technology (hardware capabilities go up and costs come down, or alternately the software is improved to do more with the same hardware) and the price of HIVE increasing (meaning resources people are willing to devote to operating nodes likely increases)