I'm ok with their interactive editor being written in SPA style. Most of Wikipedia's billions of users aren't editing pages though. For them, WP should focus on rendering speed which means lowest complexity possible, which means keeping the serialized DOM technology.
The edit view that most power users use is definitely not an spa.
There are multiple "editors" in use on wikipedia et al. Visual editor and mobile edior are more SPA-ish, however the old fashioned <textarea> full of code, editor is still very popular among users.
Disclaimer: work for wikimedia, but not on stuff related to this. My opinions are my own.