Let's be honest.
Mobile phone makers are now looking at heatpipes (some already use them) to cool the CPUs
No matter what you do, it's a tiny chip, stuck in a phone case and it's going to get HOT.
In a perfect world your phone would be your only computer, but heat is a killer.
We are reaching the limits of how much heat a phone can deal with.
Shame..........
We may get to a point where we get a new CPU every 2 or 3 years so maybe the iPhone 8 will get the same Axx CPU as the iPhone 7s with a new CPU as an incentive to purchase an 's' model along with a new color option. Frankly, just like PCs having CPUs that more powerful than necessary for most users, we're reaching or have reached that point with smartphones. The A9 is a beast of a CPU which should be more than most people need for the next 3 years or more. Even the A8 has more power than what most people need for all apps and games currently available and most in development. Developing CPUs with materials/designs that can deal with thermal issues may make the product too expensive, at least for the near-future.
Anyway, these targeted pop-up upgrade ads are definitely a sign that 6s sales are not as robust as Apple had hoped for. Welcome to a fully mature smartphone market where Apple has to to turn to tricks that previously Android phone makers were known for. I think there's a good chance that Apple will reduce iPhone prices next year with a 32GB iPhone 7 starting at $549. Maybe $449 by the time the 7s arrives.