There's something in that, but doesn't segmentation make sense purely from a pricing perspective?

There are many people who would love a Mac but don't have a ton of money. Apple don't have to slash prices to meet that demand, the current Air was already doing OK. But they're actually moving in the opposite direction, raising prices.

And there are many people who like Macs and are prepared to spend a lot of money on one. Why limit their options to a very narrow range of models?

(I guess the cynical answers would be, low-end users aren't worth much, and high-end users will upgrade frequently anyway so there's no need to sell them a machine with headroom for future expansion.)

Well, segmentation or not it's certainly worth making the Pro light and portable if they can achieve that without compromising its power too much. They seem to believe they can, or at least get close enough (I mean perhaps a larger/heavier Pro could have run to 32GB while maintaining acceptable battery life).

Whether it would be better to make a cheaper Air than the cheapest new model is a separate question. I'll just say that Apple has always gone the premium route (and the limited-range-of-models route) and there are advantages and disadvantages to that.