I would absolutely send him an email to the effect of
“Per our multiple verbal conversations, this is just to serve as notice that, in my professional opinion, your refusal to allow me to upgrade a system at risk of multiple security vulnerabilities on a platform that is no longer supported is a risk that you are choosing to accept against my advise.”
with a list of known major vulnerabilities attached if possible.
That way at least if this comes back to bite the company on the ass, he can’t say “Well he never told me this was a problem!”
Let’s do it slightly differently, let’s make the mandatory retirement age for political office the median life expectancy age for the entire country. If the politicians, etc can manage to make everyone live longer, they can hold office longer.
Similarly, take away their separate and different medical coverage and put them on the same Medicare system everyone else in the country has to use.
BitWarden provides some encrypted storage on their paid tiers. I think it’s very small, like 1GB, but it’s E2E.
Apple iCloud storage is actually E2E too if you turn on Advanced Data Protection. (Note that not all iCloud features are E2E, like email, for example.) And the price is pretty comparable too. Naturally this works a lot better if you’re on a Mac, but just FYI.