No, no it's not a huge feat for a company that controls the hardware, the OS, most of the apps, 100% of the apps that get installed on it, and all the integration in/out of the phone. It is a small feat for 5 years worth of "support". How much has really changed between the iphone 8 and 5s from a software feature point of view? The iOS has changed each year (often for not much good reason) but the phones ship with 90% the same features. I'm not talking about "well, they changed the API and now the CODE is different".
Look at Microsoft and Windows...how well Windows XP lasted for 14+ years supporting 3rd party apps, 3rd party hardware, etc. Windows 95 and 98 lasted for eons, too.
I'm not making this an Apple vs. Microsoft argument...just citing an example.
It really, really, really burns me that I buy an iPad/iPhone and unless I update the iOS every 6-12 months, I find apps that won't install. It burns me even more that, for example, I have an app 1.0 that is designed for iOS 8.0 and I install it on my brand new iOS 8.0 iPad...then I upgrade to iOS 10.x years later (while 11.x has been out for a few months) and the app has an update to 3.0 but the app is now requiring 11.x and my iPad only upgrades to 10.x! Why can't I get version 2.0 of the app that still works under 10.x?! It's a generalized example, but I've encountered this numerous times. I'm a "if it ain't broke don't fix it" person with technology...so I do not update iOS every few months like a majority of the clueless public.