All the arguments I've seen, security, developers, etc. are all mute points when you consider it's the user who is choosing what OS they want. When they choose to install an older OS, they do so for their own reasons (performance, usability, reliability, support for older apps, etc.) not for security. If a user wants to use an older OS, it's on them to employ their own security best-practices and download older software that still supports their chosen OS version.
All of what I said applies to macOS, Windows, Linux, etc. It could also apply to iOS, if Apple signed all versions.
Ensuring users can't downgrade effectively "traps" the user into Apple's decisions. When Apple decided your purchased shows/movies were now going to be greeted by an ad for their streaming service, you were forced into accepting that. You have no ability to reject their decision to show ads on your device. Ditto for Apple Music being shoved in your face when you try to use your iTunes Library.
When Apple uses more complex software, and your device performs worse, you're forced to accept that. You're essentially being entrapped into buying a new device every few years. And no I'm not saying that "planned obsolescence" is a thing, I don't believe Apple is intentionally making the software more complex just to ruin older devices, I believe that the software is naturally growing to be more complex, and Apple does nothing about the fact that you can't downgrade, because it ends up benefiting them. They're not making decisions to make your phone worse, they're not just making decisions to make it better. The default action (not changing their signing policy) benefits them.
Remember, the iPhone is your device. You bought it. You own it. You should be allowed to do whatever you want with it, whether that's running the latest OS, or installing an older OS and being able to play older games, or whatever you want.
It's so tiring seeing so many people defend the decisions of companies who are trying to take ownership away from the devices we paid for.