Nikolin an extension can add just a toolbar. The same extension doesn't need to add a new formatting engine. The default Markdown toolbar can easily be disabled by another extension with just one line of code.
We added a simple markdown toolbar to the markdown extension following the huge number of requests to have at least a basic editor out of the box.
I think the idea is that with "normal" text and autolink you can already go a long way. Then on top of this we add two very common formatting methods: Markdown and bbcode. And we also added a toolbar to help discover some of the markdown formatting.
Something simple we could add is a little help icon saying "This is markdown, here's where you can learn more about it" so that people know what to search for to learn about more advanced formatting.
If someone has a suggestion for a replacement library for github/markdown-toolbar-element, we're certainly not opposed to give it a try. We probably just won't develop something custom for the default toolbar. We prefer to leave those more advanced features to community extensions.
