I'm sure Disney did the math. My guess is that they make more of a profit from the many people who end up spending the $20 outright, compared to the larger number of people who would just pay $7 for the upgrade. If you do the math, and assume that the premium ones cost the same amount to make as the regular ones, you then find out that Disney would have to sell approximately (20 - Disney MB Cost) / 7 times as many upgrades as they would outright purchases as they do now. For example, if MBs cost Disney $6 to make, then Disney would have to sell twice as many upgrades at $7 than they would extra magic bands at $20. If they cost $3 to make, then it's about 2.5 times as many.
Are you more than twice as likely to spend $7 to upgrade to a premium band versus spending $20 to have a regular one and a premium one? For our family, we're already likely to all have the premium bands, so Disney would just be losing money on us if they offered us to upgrade for $7 instead.