I find the time it takes to swap to a new phone is significantly longer than the time it takes to get a new battery. You can drop it of any time in a year long window without issue and no need to do anything specific when you pick it up let alone decide on which new phone you want etc.

As to labor costs, people get really good at swapping batteries on popular phones and their hourly rates are low. Even Apple is only charging 70-100$ based depending the model.