As a blogger, I think having a WYSIWYG editor to be very essential, whether it is TinyMCE or any other. So, even if TinyMCE might be on its way out, it's important that it be replaced by another.
I don't think expecting users to move to Markdown as a replacement to TinyMCE is reasonable.

That being said, having Markdown as an addon would be a great addition because then basic things like bold, em, or lists because a quick thing to do. It would definitely speed up the whole writing experience.