Buying iPhone 7 or 8, ie a one or two year old phone, practically guarantees that you'll hit the end of the road with iOS major version update availability one or two years earlier, which means you'll have to buy a new phone one or two years earlier. Not sure if the $100 or so that you save is worth losing a year of updates. In other words - adding $100 or so buys you a whole 'nother year or two of usage on your current purchase.
If they offered new phones for a lower price but with the software update support lifetime that matches the flagship phones, then they might have a killer product!