Who sends sensitive information by email? Are you joking? I can ruin your life and take away all things you hold dear if I take over your email. It is a nightmare. Just about every account or service , commercial, school or government use email as a way of recovering access to your account.
It is so bad one of these days I need to ask HN properly why all these tech companies screw this up royally badly. There doesn't go by a day where some app or tech company sends by email some aspect of what I have been up to by email.
Perhaps you mean strictly from the perspective of what proton can protect which makes more sense but in general you can't be more wrong on this. Brand new apps from fresh startups rely on email all the time.
As for proton's value, it is pretty simple, you are not the product you are the customer. I have helped technically illiterate people start using it and they love it. Few weeks ago one asked me how to recover their yahoo account, i told them if they exhausted all the automated recovery options they are out of luck and gmail is the same as well. With proton, I emailed their support from a separate account and was able to talk to a human within days and get it resolved (you will never talk to a human with gmail because you are never worth the support cost). They abandoned their yahoo as I did mine for the same exact reason many years ago.
Forget the encryption, that's just polishing turd. Given the criticality of personal email, I need a company that has clear business interest to look after not just my privacy and email security but me as a customer. The stories of entire businesses ruined because google took away their account access all of a sudden is too many the ones that get posted on HN and twitter are the minority that know the right people and can make enough social media noise. There are life long contacts that will never be able to reach me again unless I hire a PI to find them because of my lost yahoo lol. They suddenly think your IP and user agent are weird and you need another device you used in the past to allow the login despite a legit password? Whoops, security security you lost your account now lol.