Apple does not give you the freedom to use old versions of software. You can't downgrade your iOS version. You can only upgrade apps, and iOS only lets you download the latest available version of an app for your version of iOS.
Need to do a device reset? If you have managed to keep any apps on old versions, you'll get upgraded now. And you can't even save APKs and sideload because of Apple's developer signatured and certs.
Old versions of iOS get some, but certainly not all security updates.
And most importantly... so, so, so many apps, from banking to navigation to music, insist on checking your app version and blocking usage with a massive dialog box if you're a couple of weeks out of date. It's incredibly obnoxious, especially when some of these apps update 2-3 times a week and each update is >100MB. I try to use those services from a web browser these days, but Safari is so limiting you can't even reliably block ironic "download the app" banners.
And that's to say nothing of actual cellular advancements, like new LTE bands and 5G and voLTE, which older phones understandably can't get... but which certain phone providers use to kick old phones off the network. For instance, AT&T no longer allows any activations for phones not on their (very limited) voLTE whitelist, since they shut down 3G.
Like OP, really wish I could just use the 2016 SE for the next 10 years. But even though the hardware is capable the software industry has pushed it into the "legacy" bin.