Iphone 15 upgrade

So the bottom line is the only way you get this phone free is trade in your old phone (i have a 12 pro max) and i would have to switch from a 55/mo plan to that 100/mo plan ?  Seems hardly worth it as i could just finance it cheaper through tmobile for probably 25/mo for 24 months after my trade in so i am now at 80/mo instead of 100/mo and that would only be for 2 years and not permanent