They probably could do it in an update, but the devices where support has been dropped haven't had firmware updates in 7 years (and that was a certificate update, the last nontrivial update was over 10 years ago), so I guess they don't consider restarting firmware development to be worth it.

Amazon can and already did this for my circa 2015 Kindle. I think it's just a lack of will to do so for devices even older than that.