Yeah I know you get the 3% but forfeiting the trade in value is a huge penalty. For example someone upgrading from an 11 Pro Max to a 12 Pro Max would get a whopping $540 credit from Apple or over $700 for a good condition one on Swappa. That just entirely evaporates with iUP.
How do you figure? I think you're mixing up the Apple Upgrade Program with some of Apple's other "trade in for credit" options.
On the Upgrade Program, you can trade in your phone for the newer model after making 12 of the 24 payments, so you're guaranteed a trade in value of half the cost of the phone. It's not a dollar value like Apple's other trade in options, like if you trade in an old Apple Watch or iPad for a credit that's calculated based on the serial number of the device - it's literally forgoing half of the scheduled payments. The Apple Upgrade Program is not the same as the other options of trading in a phone you own outright for credit - it guarantees half the cost of the phone.
Now, could you possibly sell the phone on Swappa for more than what you paid? Sure, that's possible, but it's more of a hassle, at least for me, and I've done each method multiple times over the years.