To be fair, in that sense, they should allow people to downgrade to a version that works best for them. Sure an old device might not be expected to perform well on the latest OS versions that is years newer, but it can work fairly well on an older OS version. Sure, it would be with various limitations in the sense of what apps are supported and all that), but if a user would find that to be a better experience for them, that's certainly an important factor to consider.
True but then apple carried the liability for security holes in the old OS versions That's the major flaw in that argument. Then someone gets hacked and sues Apple because Apple let them downgrade to a version with known security flaws.
It's a no-win scenario for Apple here. People complain their 5 year old device is slow, or people complain that Apple exposed them to OS security flaws letting them downgrade.