I think there are other cases. An auto-update that goes wrong could still cause wide ranging issues for non-365 Excel users. And if updates are disabled (or MS

I think there are other cases. An auto-update that goes wrong could still cause wide ranging issues for non-365 Excel users. And if updates are disabled (or MS goes away), we can add yet-to-be-discovered security flaws to the mix. Neither of these would impact all (or maybe even most) users, but I think it'd impact enough to make a dent. In contrast, Office 365 is probably less susceptible to such flaws, trading them for others.