If 5g is reasonable to run on existing spectrum (I think this is generally true, but I don't know if mobile devices are configured for it), and it allows for more concurrent use (this is the promise, yes?), and if there are sufficient deployed devices to make it useful, refarming the spectrum to 5g would be useful for improving user experience in the situation described. Most carriers already went through this with 3g and LTE.

You could invest in more 4g infrastructure in theory, that usually means adding more towers to shrink the cell size in congested areas; it's often the case that tower density is limited by municipal agencies rather than carrier budgets.

https://www.zdnet.com/article/5g-needed-to-ease-4g-lte-conge...