> Unpopular but very probably true fact [...]
That's not a fact. That's a conclusion based on the premises you put forth. You should put the conclusion on the bottom of your post. Now it is akin to a forecast or fortune-telling. There's no need to resort to such strong language in an otherwise decent post. Your premises don't need it, and your argument doesn't need it either. If you worded your post slightly less bifurcating it would've been an excellent post without sparkling the controversial discussion.
As for your premises,
> Email leaks metadata. [...]
Agreed, but
> stuff like subject lines are simply content.
Its optional content because if sender wants it to be gibberish or deceiving, that's possible.
> * Most email users get their email from a website. Unless you make them install something on all their computers
That's a matter of RFCs and implementing the new technology in client and server. If we follow your reasoning, we'd still be sitting in the previous century in old standards "because the poor users must upgrade their software."
For the other premises (such as for example "It's default-plaintext."), can you demonstrate it is impossible to fix these issues? Because I don't see that being taken into account. What I see is: "let us break backwards compatibility and switch to this here which is better" but without authority to do that for a significant amount of users we're left with the practical situation of a chicken-egg problem of a defacto standard. And could you actually directly quote from the article what and why is not feasible instead of a general dismissal? The latter feels like a rant the former feels like an in depth discussion. Someone could've written a rant like that about DNS (because IPv4), or about TCP because BGP, but what we need is solutions. IPv4 is an interesting discussion because of IPv6. Its a recent, significant example of the chicken-egg problem of an inferior standard, backwards compatibility, and how to make the switch.