How else are you going to give your front end devs work to do?

most users would like to decorate their posts with bold underline color whatever and would not want to learn markdown

Users who don't wish to learn markdown can simply spend two seconds hovering over a "Markdown guide" tooltip at the bottom of the textarea whenever they want to do that. They'll still end up saving time from the UI actually being responsive instead of loading 50 different JS libraries just to render a textbox.