actually they do have, a samsung unit swapping. But, you need to visit their physical store for the evaluation of your phone which i think this time is quiet inconvenient and you need to add sum depending on the market value of your phone and to the phone you want to upgrade.
I hardly doubt that will ever happen. As producing the new 5G model cost time, effort and money. They can't just trade it off freely. A trade discount could be an option like what they do with S & Note series. You have to understand people are paid to create this technology. If its given for free then where will their income come from?