I think something that is missing from this is that it is no longer safe in this time to use old software. It is an existential threat to your own information, which is increasingly linked to your personal device like a phone or tablet. You will be hacked by a drive by exploit, or a mutant ad network. Its no longer about risky behavior, simply existing today online with old software can put you at grave risk.
How long should apple support these devices? 5 years seems reasonable, and very few vendors of hardware offer anything like this.
I agree it sucks, and there is no reason a 5 year old ipad needs to be dead -- but in context this is fine. If you had a 5 year old car that allowed random drivers to stab you in the face remotely, people would replace their cars and not complain that this was a problem.