Yet the only alternative means that Apple won't fix all (already known and either already or soon to be exploited in the wild) vulnerabilities that your device has with the older OS, even high severity ones. Which might be fine for a media consumption iPad but is more problematic with iPhones when many people use that as their main or even their only device and have their entire lives on it, so to speak. I have OPs SE 2020 as well but with iOS 17 and I can tell you from my experience that it was already slowed down with iOS 16 and hasn't changed with 17. iOS 17 merely put another nail in the coffin of the dwindling battery life but in that regard that iPhone was doomed from the start.Keeping automatic updates on is a sure-fire way of obliterating every iOS device they have with 100% certainty of the result.
The Pro iPhones do not have this issue to the same extent. They came with bigger batteries and more memory and all Pro iPhones currently on iOS 17 run as well as you can expect them to run iOS 17... which is not all that well, but that goes for the latest iPhones too. As we all know Apple likes to use their customers as beta testers for the first few months.
The problem here isn't inherently the upgrades, it's that the SE line of iPhones is cost-optimized to hell and back. Apple re-uses old hardware lacking the latest power optimizations, combines that with a smaller battery and throws it in an old design with power hungry older display panels and probably doesn't care to optimize later iOS releases for older SE iPhones too much.
My SE gets the job done today just as well as it did on day 1. Which is not very well but I got what I paid for, a cheap iPhone that is still usable going into its fourth year and receives all firmware and OS updates. That's what I bought it for, to get more than the usual 2-3 years of upgrades that Android devices offered at that time.
Apple only guarantees updates in a timely fashion for the latest major version of any of their OS'. So yes that is one downside of using a device that cannot be upgraded anymore.Between 30 November 2023 and 11 December 2023, Apple left iOS users that can not upgrade to iOS 17 with actively exploited vulnerabilities in their devices.