But styles are one of the things that set Scrivener apart from the 1,001 Markdown editors out there! If we were to hypothetically get rid of all of that and just do like everyone else does, it would be a massive downgrade in my opinion.
That’s just styles (as a formal feature), there are other things more loosely related to formatting that you can’t do in most text-driven Markdown writing tools: revision tracking, freeform multi-colour highlighting, sensible annotations and sidebar comments/footnotes (and having those footnotes sequenced for you automatically), links that are purely editorial, and really the possibilities are endless and freeform once you switch to the perspective of looking at “formatting” from the angle of it being a writing and editing tool, rather than something you need to use to make things look a certain way.
Besides you already essentially have the choice you state you want: it’s just not a toggle somewhere that would destroy your project if you turned it on without realising the implications. Instead it is a matter of simply not using what you don’t need. Turn the Format Bar off, set your preferences to work more like a text editor, and get back to work. 
At any rate, this has been discussed many times before, and if none of the above is compelling, feel free to search the forum for feature requests for “syntax highlighting”, “markdown mode” and so forth.