I installed LineageOS 16 (= Android 9) on an old Galaxy S5 of mine today, just for the heck of it (I had always been curious about it).
It works perfectly. Quick, responsive, no stuttering.
So you can run the newest version of Android on a phone from five years ago and it does so just as well as the latest, most expensive model. So other than getting a better camera, there is no reason to buy a new phone. In some ways the old phone is actually superior because you can replace the battery (costs ~$10).
It's all a scam.