Who I have the same questioned this solution

Let's the phone was less than $5 out of that $10.  In 4 years...  48x5 = 240.  So you saying that's enough for them to recoup the costs?  Sorry, but I don't think so.  Also you used an upgrade for a smartphone and swapped out for a basic phone.  So where are they going to recoup the difference?  Also you agreed to the terms which were printed out on your receipt which you had 14 days to read.  If you bought it online you still had 14 days, and it forced you to pick a data plan in order to do the transaction. 

you are grandfathered into an older plan and your device upgrade for cyyrent plans for the price difference.