But I don't agree with your conclusion that they need to be protected. If I want to hammer in a nail and accidentally hit my finger that's not the hammer-manufacturer's fault or responsibility. So why should it be the email-client's fault/responsibility to make sure I don't send my private keys?
If people refuse to learn how to use a hammer they will keep hitting their fingers, if people are refuse to learn how to use their email-client they will keep sending their private keys to bad actors.
But I do believe that UX needs to be designed according to the principle of least surprise, and those in the know (i.e. we/us), need to put in guardrails to keep people safe as they get on with their day.
If your hammer has 4,500 different everyday functions, 4,400 of them posing a danger to your fingers & bank account, but hammering nails is necessary for your job, for your kids' school stuff, and to interact with your government, then that's a closer analogy.
Software, especially an email client, is significantly more complex. Many people don't have the time, desire, nor ability, to learn how to use it safely - and the usage of it is generally forced upon them.
Let's make things safe by default.
A lot of people, most even, have no idea what is happening on about 90% of their screen, about 90% of the time. And they're used to being forever in the dark about what their machine is doing and why. They expect it.
They know they're writing an email and they can see the box they're typing into, but other components, like the status bar, title bar, and attachments field, are outside their sphere of perception. At least or especially when they're not actively thinking about attaching a file intentionally, say.
It's like a serious chess player, who sees the whole board and thinks multiple moves ahead, compared to a novice who learnt the rules yesterday and can only see the horsey piece and the pieces immediately surrounding the horsey piece.
These users have a checklist approach to using their computer:
Step 1: click this
Step 2: that happens
Step 3: Did what I wanted to happen, happen?
Step 4: there is no step 4
The professor who didn't realize there's a "fullscreen" button when showing videos to the class is the same one who can't perceive the attachments field and will send a file called 'passwd'. And if the mail client puts up an alert that says, "Are you sure you want to attach and send a file called passwd to [email protected]?", they'll click 'Yes' and get on with their day.