Monospace is the more sensible option as an editing medium for symbolic content. It's in having "WYSIWYG" editors that do everything in one modeless widget that we start getting this particular kind of feature creep.
And considering how awful it tends to be to navigate a complex word processing task in Word or LibreOffice even now - mystery-meat selections abound - I'm not sure this is the right feature to support all of the time.
Proportional fonts are more aesthetically pleasing and legible (the entire reason for their existence).
When I'm editing "symbolic content", I like it to be easy on my eyes. If I'm constantly re-reading what I've just written, I appreciate the added legibility.
(With the exception of heavily punctuation-based computer code, of course, where monospace results in greater legibility.)
And considering how awful it tends to be to navigate a complex word processing task in Word or LibreOffice even now - mystery-meat selections abound - I'm not sure this is the right feature to support all of the time.