I suspect Apple has traded higher display/GPU power consumption for lower components elsewhere ( better CPU , chipset, lower power radio infrastructure, lack of fan power draw , etc. ). This new model is probably in similar battery lifetime range as 'old' MBA 11" is now.
Power is in the good enough range if not looking for laptop to serve as charging dock for other battery powered (or just bus powered ) devices.
I think the "good enough range" as you put it is the key. We aren't talking about the extreme usage scenario for a mass market device. We need to focus on normal usage and the spikes in normal usage. Once a laptop has reached a certain level, the battery life is good enough. Example. I frequently bring my laptop to a coffee shop to work on the weekends. My last laptop was an HP Specter, purchased in early 2013. Slim, sleek, relatively light weight. But it got a little more than 3.5 hours of usage doing word processing, emailing, surfing the web, and a bit of Excel. So I could go to the coffee shop and have lunch while working and kill the battery in one sitting necessitating a return home for recharge. It wasn't a big deal, but it was an issue to work around.
My current laptop, purchased in late 2014, gets a battery life I have no idea what it is. It estimates 13 hours if I set it at eco boost. I don't know if this accurate. I will never put in enough hours in a 24 hour period to figure this out. If I have to put in a 20 hour day, I will be either at the office (and plugged in) or at home (and plugged in). If I'm traveling, I won't actually be able to work on a laptop while I'm in cabs, waiting on security line, or boarding air planes. I'm sure I won't ever have to (A) do a 16 hour flight and (B) have to work on my laptop during most of it. I also have an iPad which would be with me on any long flight and would provide recreational usage for nearly 10 hours.
There is a point where battery life becomes irrelevant for most usage. Yes, there are folks who use their laptop as their big battery to recharge their phones. Those phones are not iPhone 6s because you will either get all day life from your iPhone 6 or you will get all day life from your current modern laptop. You can't, in any normal usage, be actively using both devices so much that you kill their battery in one day (one major exception, you are using your iPhone as a hot spot to run your laptop). Apple should design and market its laptops under the assumption that the user will have a modern smartphone (iPhone 5s at least) and/or an iPad with them. It is a fairly safe assumption I suspect. So while battery life is key, they don't need to assume that this will be your one and only device over a 12 hour period (in which you don't have your power cord).
I hope Apple kept the goal of liberation from battery life in mind when it made the next Macbook Air. But it doesn't have to go to extremes for the M Air. The M Pro is the space for the more hardcore mission critical stuff. That differentiates the two lines in meaningful ways. And you have to admit the lines were not well distinguished in the last two years. The price point was too close, the performance was too close and the form factor was too close.
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My prediction: This will be a low price ARM OS X netbook that only runs apps from the App Store. No third-party apps, no Intel emulation, just apps recompiled for an ARM port of OS X and sold through the App Store.
ARM (an "A8X" or whatever Apple variant) would let Apple get their own price down to tablet levels. It would also help them lock down the machine similar to the way they do with iOS devices so Apple can focus on profit from apps, media, and iCloud services in their walled garden. With family sharing and a low hardware price, it would make this the logical choice for family members without desktop computing demands, or for a lightweight spare machine from people who are primarily desktop users.
It would be a play similar to Google's Chromebooks, but with much more credible apps.
I think you are a year or two away from this case. Way too confusing to sell a laptop that isn't OS X at this stage. Once iOS and ARM is powerful enough, this could happen. But not in 2015. Or 2016 for that matter.
