This suggestion seems to stem from a very narrow view of Markdown: that it is a way for Internet users to enter content into web applications.
But Markdown is just a way to write structured text and CommonMark is just a standard way of turning that text into HTML. Some people use Markdown for writing personal notes, others use it as an HTML alternative (like HAML). A tool like Pandoc can even convert Markdown into completely different formats. In these contexts, “safe” Markdown isn’t just nonsensical, it can be counterproductive!
I was under the impression that CommonMark was intended to be a standardized Markdown that all applications could use as a baseline before adding their application-specific customizations. But if I’m wrong and its really intended to be a standardized Markdown for online text input, we should definitely make this change.