Anything beyond Mavericks was unusable on my 2010 13" MBA. Things like Mission Control would take 3-5 seconds to respond, whereas with earlier OSs it was pretty much instantaneous. Battery life was barely 2 hours and fans came on much more frequently than before. And I always did clean installs as well, so it wasn't due to clutter in the OS from upgrades.
Of course that doesn't mean future upgrades will kill the MacBook. My MBA was one of the last Core 2 Duo Macs so that likely played a role with optimisation, but at least in my experience OS X tends to get heavier by a bit in every upgrade.
Maybe that was because of the amount of memory you had. Or because of the graphics performance of the MBA, because everything beyond Mavericks is using the GPU for UI rendering, as far as I know. Or did Mavericks already started using the GPU? And be able to use a more "powerful" GPU (as long as we can use the word "powerful" for the integrated GPUs), developers stop to think about an outdated system.
I remember, that the first OS X update I got, after buying my MBA early 2013 (i5, 8GB), had improved a lot the speed and responsiveness of the system. And until today no OS X version released let me think that my MBA was too slow. It even gets better and better and I never made a clean install, always just updated. Even now, where it has a water damage and with that a throttled CPU (IA:0,8 GHz, GT 0,2 GHz, fixed, no more turbo boost; Geekbench 3 32Bit benchmark single-core: 824, multi-core: 1695), the UI is still acceptable, some things like mission control, launch pad are perfectly smooth, others like animated menus and switching spaces could have a little lag from time to time. The overall processing performance is like a time travel, but still usable (I’m using OS X El Captain). Finally, that’s impressing that a 800MHz CPU is fast enough to still have a usable system and a little training of patience for installing / starting apps.
My experience (with OS X and iOS devices): when you buy a system, you will be fine with the upcoming OS updates for at least the next 2-3 years. For OS X maybe 3-4 Years. And if computer system becomes outdated (but still usable) after 3-4 years, that will be ok, IMHO.
BTW., I think that when the time comes where the Macbook 2016 will be to slow for further OS updates, it really doesn't matter if you have the m3, m5 or m7 version.