I don't see a growing size as an "upgrade" any more than someone getting an SUV over a compact car would consider an SUV an upgrade rather than a preference. So I don't think apple will make each successive generation ever larger and require you to upgrade size to get the new specs.
More likely, that is if Apple decides to introduce another large size, it will be a MBP type situation where two sizes are upgraded together and people have a choice of which to buy. I will agree though that 3.5 will go away because it is a different aspect ratio than the 4 and 4.8 (if the 4.8 even ever happens as that's just conjecture with no evidence).