Well, by the book and its origins, there is no improvement just because in Markdown you can use any HTML. Markdown itself is part of the try when you follow the path of its history and when marketed as a subset, it means (and that approach may be valid), let us go back and reduce to the early set of HTML tags.
I wrote enough basic HTML tags back in 1995 to say that writing _this way_ is way more ergonomic than <i>this way</i>.
The thing is that HTML (and SGML) was invented, but Markdown was discovered as a set of best practices through decades of text-only mailing lists. This is something that many people found natural enough.
Sure, if adding symbols to text is enough, why do more?
It's bending perhaps history of mankind in my book a little too much, saying the author of Markdown has discovered that and not lets say, someone else the "one or other" millennia earlier.
And writing on the computer is commonly the least ergonomic form of writing at all.
