With inline LaTeX previews, we're already surprisingly close. In fact, I'd say that going all the way would be almost a step back. WYSIWYG is ultimately not an ideal editing paradigm: it wins in the short term, being easy to learn, but drags you down in the long term. I've recently started using Quora a bit more. Unlike StackOverflow, they use a WYSIWYG editor. I've found this significantly less convenient than Stack…
If you want to try it yourself: The most important commands are [ctrl]+[m] to enter math mode (or [ctrl]+[shift]+[m] for display formulas) and [alt]+[m] for a lot of math shortcuts, specifically [alt]+[m] and then any bracket to open matching size adapting brackets of this type. To enter a sum for example press "[ctr]+[shift]+[m]\sum[space]_k=1[space]^\infty[space][space]\frac[space]1[down arrow]k^2[space][space][space]". The shortcut [alt]+[p] and then [space] to choose the paragraphs meaning (section, itemize, standard, …) is also a huge productivity boost.