I like that this exists, as someone who writes everything in Markdown.
(I should note that there is a platform that offers something similar to this already: https://buttondown.email/)
But while Markdown on its own is great to see more of in the email space, I think there's room to extend the idea. A few years ago, an employee of CakeMail created this great demo of a concept that mixed shortcodes and Markdown, but never followed through with the idea: http://cakedown.alexandredeschamps.ca/
Part of the problem with that design was that it was built around specific components in Foundation for Emails, which kinda backed that design in a corner, but there's room to allow end users to custom-build components for easy access, along the lines of what MJML can do. To me, I think that idea has a ton of blue water around it. Nobody is combining shortcodes and Markdown in this way for email clients, and it could potentially be very valuable as an alternative to the overly complex WYSIWYG editors the rest of the email market uses.
I run a newsletter with 10k+ subscribers (https://tedium.co/) and I run it on Craft CMS in a way that replicates the CakeMail approach to a degree, with my own flourishes (which was convenient as I already use shortcodes for my text). It allows me to throw in ads on the fly and simply add design features to content without a lot of extra work. I'd love to see the general idea get picked up by some new email provider.
I think we need more thinking in this direction, because a lot of email tools are built for marketing, which means that editorial concerns get short shrift. I could see stuff like this making a lot of sense for the growing editorial newsletter market if it's handled the right way.