They weren't "saying things like", that was the stated purpose of these mugs back in 2000. I know, I bought five of them, and still have them. We brought them back several years. I don't use them anymore, too big to pack in the suitcases. But I just wanted to emphasize, those were and are "forever" mugs.
I know this isn't the Official Mug Thread, but I would imagine Disney's official line COULD go something like this: our policy when we sold those mugs was to honor refills forever. Due to changes in costs and business models, we are no longer allowing infinite refills. If you are now unhappy with your purchase, we will gladly give you a refund of your original purchase price. If you want, you may choose to apply that refund to a mug for this current stay.
More practically, they don't want to get into confrontations and make people angry over something as simple and cheap as soda. They're not going to go out of their way to CAUSE a confrontation with you. Instead, they're just going to change their policy by adding barcodes (or RFID tags, or whatever) until you CAN'T use the mug anymore. At such time, it will be up to YOU to make a stink about your "forever" mug, and escalate the issue as YOU see fit.
Personally, if I had one of these older mugs (my oldest mugs date from 1996, which I remember as being from the "forever" time, but don't say so on the mugs) and I really wanted to make a deal out of this, I would try to deal with this before the whole barcode / RFID tag cuts me off. I'd contact Disney ahead of time, saying "I've been to WDW numerous times, and I have these here mugs from X year, and they were sold to me as "forever" mugs. I hear you have a new barcode (or RFID) tag system. I have a trip planned for next month, spending lots of $$ on tickets, resort reservations at X, etc., and before I pack my mugs I'd like to know how to go about getting them refilled as promised." Then, having put the ball in their court, you at least give them the chance to offer you something to "settle" this issue for you.