If they did it right, it might even be a good thing. But there's not really much desire/push for it, because spam got "mostly solved" well enough.

You'd want it to be a feature-add (if enabled and used, your bank can send you secure statements direct to your email box that nobody else can read, etc) so you actually gain something; but then you have the whole "Google and Microsoft have MITM themselves on almost all email" problem anyway.