An older phone may simply not have the hardware necessary to run the latest version of software. The phone will still work. You are not being forced to do anything. An older brand X phone will probably not do 4G LTE. Or maybe even 3G like a 2014 Q50. They all will need to be upgraded at some point.Ah yeah they cut support for both hardware and software after set periods of time, Apple does that a lot on the software side in particular
to force people to upgrade from older devices.
Not a big deal if you are upgrading your phone every 2 years or so but If you hold onto your phone for
4-5 years it feels like your being black-mailed into upgrading when you have a perfect functional device.
Until very recently I used the latest Apple Airport extreme first as a router and then simply as a WiFi access point. It has been out of production for years but it is still being supported. Not with features but I got a security update about a year ago. It still works like a charm but I went in a different direction with some Ubiquiti stuff. I needed more flexibility and options.