From a business point of view it makes zero sense for Disney to allow refillable mugs in the parks. Not to mention it would be a logistical nightmare. The coke freestyle machines would be a non-starter as the syrup containers are extremely small so they'd need refilling every half hour. Even if you go back to the standard fountain machines, they occupy a ton of space for the compressor, syrup boxes etc. As a youth I worked in a well known fast food chain and the Coke syrup boxes came on a pallet. If you can imagine the size of a palet and make it into a square cube that was the size of 1 syrup box, just for coke, it would last a day or two at best. Now multiply that by the ~12 or so drinks you see on offer and you quickly realise you cant just drop these in small spaces around the parks.
The cynic in me also says the resort refillable mugs are intentionally made to have leaky lids to stop people carrying around a drink in them at the parks so you'll buy a drink instead.
Personally I'd rather see more of the newer filtered water bottle refill stations around the parks.