We were in a similar boat.
Traded in daughter's iPhone 12, received $830 (in 36 future monthly credits) towards a new iPhone 15. For us it wasn't a big deal as my daughter will most likely hold onto the iPhone 15 for at least three years barring any accidents, but she takes really good care of her stuff so I'm not worried.
Wife is coming from a 13 Mini which she was receiving monthly credits from a prior 11 Pro Max trade-in. By upgrading early we do lose half of the credits from the 11 Pro Max, but receive $830 future credits for the 13 Mini towards a 15 Pro. We figured this was worth it as she's been disappointed with the cameras on the 13 Mini and waiting to recoup full credits on the current Mini would mean less trade-in value on the Mini at the 3yr mark. We had to buy out the Mini though for $400 before we could trade it in and I was fine with that. Usually my wife keeps her iPhone for 4-5 years. The 13 Mini was the exception and really not a good fit for her -- too much compromise in exchange for a lighter phone.