As the number of [postal] mailed letters drops from year to year,
I wondered if the icon just can't be associated with the letter
The paper envelope says “letter” not “postal mail” or “mailed letter.” Letters (and envelopes) existed before postal mail existed and will exist after postal mail stops if it ever stops. Letters can be written on paper and hand-delivered or written with a computer and sent via the Internet with no paper involved. They are still letters.
Similarly, essays can be written with a pencil and paper, or typewritten onto paper, or written with a computer and published on the Internet. They are still essays.
Short stories, novels, legal briefs, etc. — none are dependent on paper. Just like letters.
Movies didn’t stop being movies as they moved from videocassette to optical disc to cloud storage. The system that grabs a filmstrip by its sprockets and moves it across a light bulb has been replaced with a digital system that does exactly the same thing. There is even a light bulb.
So the key aspect of email that the envelope icon is describing is that you are sending letters. Not 140 character SMS messages or tweets. Not koans. Not novels. Not interactive chat.
The icon is working perfectly. Email is broken. But that is another story.
Not at all the same.
Letters are conceptual and timeless and universal. The floppy disk is mechanical and short-lived and obsolete for many years. The concept of “Saving” is also short-lived. Most computing systems today just auto-save every 2 seconds.
The floppy disk is a control element like set in stone. It was, is and
will be there to save.
That is not at all true. The floppy disk Save icon is Microsoft-specific. It was not even universal to tech whereas letters are universal to the human race. I have personally never used the floppy disk icon, and only seen it in UI/UX articles.
The envelope however isn't really mandatory. You can replace it with a
How many letters have you sent by writing on paper and then folding the paper into a plane and throwing it? Letters in envelopes is inconceivably more popular.
a pen (for "write mail")
Pens are not just used for letters. And in a digital context, they are almost exclusively used for drawing and painting, not writing text, because the typewriter keyboard is much faster for text.
So I think the answer to your question is that the letter/envelope icon and the Save/floppy icon are not the same and the letter/envelope icon is going to be around for the foreseeable future.