
"perhaps such cross-platform tools just aren’t needed any more"
I beg to differ. Cross-platform mail security is indispensable, or, at least the ability to send mail securely from any platform to any other platform is.
One must not consider that, because Facebook users might mostly write to other Facebook users, it is not important for them to be able to write and/or receive secure mail to/from other platforms.
Because the alternative is that, to send secure mail, you'd have to be on FB and send to a user on FB.
And having Zuckerberg in charge of secure mail just cannot be acceptable.