I don't think you should wait until the device gets no longer supported, as the trade-in value will considerably dip after that. Currently you are getting the device free or if there is some hidden cost, an almost free upgrade.
As per me, you should do the trade unless you want to wait for iPhone 12 or something. Apart from the amazing upgrades mention above, remember the new iPhone SE will have better resale value than iPhone 7 in another two years or so. You will kind of have a double profited deal!