The fact is, iDevices are like how PCs were back in the '90s, where the software outpaces the hardware. Now with the Air 2, I think that is where the sweet spot will be for years to come. It has the hardware to keep up with software update for at least 3 years or more. Any device lower than an Air 2 will take a beating with every major iOS release.
Yes, and not just like PCs in the 90s, but Mac laptops in the '00s. Esp. in the PowerPC era, all consumer Mac laptops shipped with way too little RAM, and OS updates quickly made those machines uber slow. However, the benefit was one could upgrade RAM after the fact, although some machines like iBooks and iMacs were problematic since the max RAM configurable was so low.
iPads have been following that trend of shipping with low amounts of RAM, but the problem now is that RAM is not upgradable or even configurable. This is a big reason why after the iPad 2, I've avoided every iteration of the iPad until the iPad Air 2 with 2 GB RAM... and then I bought two of them. They should last a while. The 1 GB RAM in the iPad Air was a real disappointment in an otherwise solid release. Those who were seduced by the external beauty of the iPad Air are already suffering the consequences, with features in OS 9 not being supported.
But all of that said... the benefit of the Macs is you can install whatever OS you want on them, security updates be damned. When I discovered my 10.5.8 installs were too much for my 1.25 GB G4 Macs, I downgraded to 10.4.11 no problem. However, on my 2 GB G4 iMac, I put it on 10.5.8 which worked decently well.
The ironic thing though is I have the opposite problem for my 2008 MacBook. I'm stuck on 10.7 Lion on that thing, because Apple won't support it for anything past that. 10.11 El Capitan works just fine on my 2009 MacBook Pro though. The two machines have the same 4 GB RAM, and the 2008 MacBook actually has a faster 2.4 GHz Core 2 Duo, compared to the 2.26 GHz Core 2 Duo of the MacBook Pro. The difference is the MBP has a discrete nVidia GPU while the MB is stuck with an Intel GMA X3100.