Bather load should have nothing to do with need to replace water or perform RO treatment. While there is much talk about RO treatment, the simple truth is the service is available in very few places (mostly select parts of southern California and Arizona) these are primarily areas with very high CH fill water, so water replacement only goes part way in solving the problem, and CH starts building again due to evaporative concentration. Even in these areas the economics of RO treatments are questionable, as the cost of the service often exceeds the cost of substantial water replacement. Cost of RO treatment may come down in the future as there have been multiple advances in the technology in the last couple of years that has the potential of greatly lowering operating pressures needed for RO membranes (from the 1,200+ psi ballpark, down to the 150-300 psi range).
To sum it up water replacement should only ever be considered for:
High CYA, High CH, high metal content if a low metal content fill water is an option (high metals caused by long term use of of copper containing algaecide, or trucked in water being used as an alternative to well containing high metal levels), or possibly some exotic type of contamination