I very rarely use the WISIWYG for PlantUML, at least the built-in XWiki. I believe that for Mermaid this is a minor inconvenience compared to the ability to use Mermaid diagrams. Third-party LiveEditor will allow you to debug complex diagrams and perhaps later it will be possible to teach WISIWYG for make Mermaid too. In the meantime, even just supporting Mermaid will give me significant benefits of using XWiki.