Not everywhere you have first party service to swap battery and usually after 3rd party swap you cannot trust water resistance anymore, therefore this tactic might not be for everyone

Also by looking at metrics of my wife phone you are basically even after about 1.5y - 2y (gets to 80%), so you loose battery life first but after that time you are net positive.

If you plan sticking to your phone for longer time, then it's good investment.