I'm aware that it's chassis is made from tough materials, and I even know what steel is. My point is that a 0.6" thick laptop[1] with the same processor boasts 18 hours of battery life as the standard capacity. Unless you're alleging that there is a 1.5" steel plate in that thing somewhere, I suspect theres some room in that 2.1" that could fit a better battery.
That space is being taken up to deal with stricter thermal, dust, moisture, RF, shock, vibration, etc. requirements. It'll run on one battery while you change the low one. I really hope you're kidding trying to draw any comparison to a Surface laptop.
[1] https://www.newegg.com/p/1TS-0025-00CZ9