Just bought a brand new 128GB 7 Plus few months back. It's still smooth phone, but it does feel warm/heating up at times when playing games. Other than that, the A10 is a great chip and I'm sure Apple will continue supporting it for at least another two years, if not longer. It is still being used in the regular iPad. So as a current 7 owner, I don't feel it being "slow" in regular use.
If you have the budget though, the iPhone 9 will surely destroy the 7 in every way, especially with the much more efficient A13. The A13 will also bring features like smart HDR.