Having used a lot of markdown and reStructuredText, I can see the arguments that the lack of common standards in markdown is a huge pain. There's still a lot of variety between markdown implementations and while Common Mark and open GFM standards are helping to a point, you still can't always count on a markdown engine to have things as basic as definition list support or HTML anchor support (linkable ids for sub-headings), and there are still some weird subtle implementation differences in those things.
There's also a lot to be said about the way that reStructuredText provides standard plugin spaces in the markup. While it may look over verbose at first glance, it is a great thing when you start to pick up that a lot of reStructuredText is based on the same plugin markup and there's a consistent way to work with new plugins. Something like that could be hugely useful to Markdown, and probably could have stymied some of the complications in standardizing things like Common Mark and GFM had that consideration been baked in from early on.
There's also a lot to be said about the way that reStructuredText provides standard plugin spaces in the markup. While it may look over verbose at first glance, it is a great thing when you start to pick up that a lot of reStructuredText is based on the same plugin markup and there's a consistent way to work with new plugins. Something like that could be hugely useful to Markdown, and probably could have stymied some of the complications in standardizing things like Common Mark and GFM had that consideration been baked in from early on.