The number 1 issue with using CommonMark here is lack Discourse specific extensions.
And the number 1 extension we need to even enable stmd.js as plugin would be some sort of support for quoting with meta data.
When I quote reply your post I get:
[quote="eanderson, post:1, topic:168"]
Below, I will create a fence
[/quote]
If I just quoted the text in the standard way I would see:
> Below, I will create a fence
But this contains no metadata about which post I quoted and elminates a core feature of Discourse. Figuring out syntax and support for something like the above in CommonMark would pave the way for a move. Without it we are stuck.
(on top of that we have bbcode support syntax highlighting hinting and some other stuff, but none are as critical)