Right to repair needs to cover this issue. Either keep devices in your ecosystem up to date, or open it up for third party support.
I am staring right now at two mobile dev test devices I have on my desk, a Samsung Galaxy S6 and an iPhone 6. iPhone is running iOS 15 released this week. Galaxy is stuck on Android 7 because Samsung don't bother providing updates to devices beyond 2 years.
It's no coincidence that my latest personal phone purchase was from Apple.
(Well, that's not _quite_ true. I have a PinePhone on the way. But I'm expecting everything I deserve for buying a beta version of a Linux phone from a niche manufacturer with _that_ one...)
Also didn't see it mentioned in your comment, but both were released in the same year (2015). Additionally, although the iPhone 5s (2013) and 6 (2014) are stuck on iOS 12, they're still getting security updates! [0] That's 8 years of support so far.
I'm still using a 6s, but planning on upgrading in the next few months. It's nice to have the option to keep using the phone though without losing support.