Here is the thing, your right, but I know people who use Apple Mail for work. Although it is simple, I don't feel it is centered around teenagers. It displays emails, it gives us hyperlinks, and it does it all in a minimal, yet somewhat confusing, UI. I agree with you claim, but I do not see the teen-focused aspect of Apple. If they did, then we would have a cringey Apple social media.
You may well be correct. But I do know Apple did not design Apple mail for me.
It seems to be designed for simple minded people that do simple minded things (hence most likely teenagers or early college students that are more concerned about how they look than how things work). While I use it, it certainly does not meet my needs.