I don't think that's fair. I think "when you SELL code", there's an implicit (and usually explicit) commitment to improve/maintain it, but not when you give it away. When someone buys me a Christmas present, I don't go to them to repair it if it malfunctions.

Besides, the Markdown license explicitly says it's "as is" with no such support promises.

I don't think you can even call Markdown users Gruber's userbase. He created Markdown because it works for him. If you don't like how it works, change it and call it something else (MarkSide? MarkIn? MarkOut?)