XML has attributes, not JSON, so in order to handle the "conversion" to a class, it is not as trivial nor as reliable as you think.
The purpose of hot reloading of xnbs is mainly required by editors, right ? Not in-game ? It would be a nice feature, but I can see many games built with MonoGame, more than editors (2 maybe 3 projects). And as within an editor, when modifying a world object or a shader, the app knows which file has been changed, it is more a thread safe content loading problem at runtime than a FileWatcher on a whole directory (which is not as reliable as it is said)