I said nothing about HTML though, did I.
JSON and XML are both difficult to edit by hand, so don't do that. JSON, arguably, is harder to edit by hand because there's no CDATA, so you have to know how to escape characters, especially newline.
Both are tree-like structures. Why would one be better than the other? Well, XML has a long history of use and development for text documents, whereas JSON does not, so right there, that argues against JSON.
Does XML suck for WYSIWYG editing? I don't see why it would suck more than, say, TeX. LyX -a WYSIWYM editor for LaTeX- exists, and it can output XHTML using a LyX-specific schema. I'm sure LyX could easily have been built using an XML format from the get-go. You've not made any arguments as to why XML suck for WYSIWYG editing -- the example you pasted surely is meaningless in a WYSIWYG editing since that kinda implies having... a WYSIWYG (GUI) editor. And surely JSON will take more ceremony to express 'bold and italic' than XML, and not much less ceremony than XML to express 'use this specific font' (where the font's name alone is lengthy enough).
All you've done is expressed a preference for encoding details that no one really cares about, and you may not have stopped to really learn and understand XML, so your preference is leading you into making a number of mistakes the most important of which is NIH reinvention. Each NIH reinvention adds a fair amount of cognitive load for the rest of us, and adds development and maintenance burdens that never go away. Please don't do this. Innovation is very important, of course, but don't blindly reinvent to innovate.