I understand your point. I'll speculate their aren't enough guests in your category to justify the inherent issues in re-activating old mugs. I think a lot of people in your position, customers close enough to drive several to WDW often, will say something like I might as well spend an extra few bucks and get a new mug.
Actually, I don't think you did get my point. It's not about the money. It's about the plastic waste.
It depends on how many old mugs Disney wants to move. They could just as easily start selling the new mugs, with the RFID chip active days, or even weeks, before the machine checks the chip. They wouldn't need a 14 day "grace" period. Maybe 7 days. At that point a CM could assist customers who are still entitled to refills. Give them a new mug, put a sticker on the old mug or just manually override the system for every fill. I'd think most guests with old mug designs and mugs so beat up they were obviously brought back wouldn't bother to fight the issue. Don't a significant number of customers pay for their mugs via room charge?
People want to know how to "beat" the system.
Paper cups are reported to allow up to 4 fills over the course of an hour. Garbage pails have to contain cups with fills remaining.
According to the FAQ linked in the OP customers not staying at WDW resorts are permitted to purchase a refillable mug. Just tell the CM you're staying for 14 days. At the end of your stay sell the mug to an arriving guest. Maybe we can set up swaps.
Disney's inventory of existing refillable mugs. Can be sold as souvenirs at WDW. Sold as souvenirs in outlet stores. I don't know if putting an RFID sticker on the mug is a big deal. They could be sold with the same sticker as a paper cups as an upgrade. Only an extra dollar gets you a souvenir mug instead of a paper cup.