I hear this same thing a lot on the T-Mobile subreddit. The thing is that T-Mobile stopped two year subsidized contracts before I even joined up in 2015. They also had two versions of JUMP at that time which allowed you to upgrade a phone either every month or when half the old phone was paid off. It depended on which version of JUMP you were on. In both cases, you lost any money you'd already paid into the old device.I think most people upgrade their phones because their carrier tells them "You're eligible for an upgrade!". That's why I often upgraded back several generations ago, and why people do that I talk to.
What's interesting to me is that people still bring up 'eligible for an upgrade'. There are no more subsidized contracts, so as a customer you can upgrade any freaking time you feel like it. You don't have to wait. Either just add another EIP (Equipment Installment Plan) up to your credit limit or pay the old device off and get a new one. Heck you can even 'upgrade' outside of T-Mob by getting your own device elsewhere and putting your T-Mob simcard (or ESIM) in to it. You don't have to wait to be 'eligible'.
So…I think what people REALLY mean by this is that they are eligible for a PROMO. And that's the difference. If you can upgrade any time you like as I just mentioned then the only thing you're actually waiting for is to be 'eligible for a promo deal. Not an upgrade.
Just very interesting to me how people phrase things. It makes it seem like they think they're trapped or something.