Notes is already a WYSIWYG editor, with a feature-set exceeding that of plain Markdown (handwritten notes, math formulas and plots, colored highlighting, etc.). In the general case, Markdown export and re-import would likely be lossy, or would have to use HTML elements for non-Markdown features. The main question IMO is if they’ll add Markdown source visualization and source editing, in addition to export/import. It could conceivably even just be export, without import.