No one can tell you. I’d guess that MD is already more widely adopted than RTF ever was (or will be). And (someone else already mentioned that) RTF can easily be worked with using scripts and basic command line tools. See for example this thread Setting created/modified dates from markdown front matter
Or all the others where people extract information from MD or transform the file’s content.
Though RTF is a clear-text format, it’s nearly impossible to script it (without using third party software like Apple’s Richttext framework), much less handle it with command line tools.
In my opinion, RTF is a dead end. Though the end may be far away.