Really appreciate you taking the time to write that. I have a sense of most of that (“defense in depth” and “threat model” are good lenses to think about such things through for sure!) but what I was trying to get a better grasp on was how much risk from automated attack was a normal person without worries of an “advanced persistent threat” taking on by using a device past EOL. Like you say, “Quantifying how much of a difference it makes is not trivial” so I feel less conflicted to know that you’re comfortable with your dad taking that risk.
I would think that the main thing at stake for a typical user isn’t just browsing history or email though but rather identity theft since a successful attacker can use the device to get through 2FA.