@DannyH (WMF) you appear to be the author of the relevant text, so I'm pinging this to your attention.
You wrote visual editor is the default editing experience on desktop, and that desktop, brand-new editors are mostly using visual editing. This is incorrect. Our wikis are fragmented into four configurations:
- For the largest group of editors, wikitext is the default editor.
- The second largest group do not have a default editor, the wiki default provides parallel links to both editors.
- Due to a bug, some wikis were set to VE-primary in the Single-Edit-Tab deployment. Note that the manager on the project gave an explicit assurance that VE was not going to be set primary without asking the community first. Staff have been either non-responsive or combative in regard to fixing this bug. It only got fixed on three wikis so far, and only after community members started writing sitewide-javascript hacks to fix the bug ourselves.
- The smallest group is... I think two small wikis ...where VE is legitimately VE-primary.
By the way, the "alarmingly low" wikitext completion rates and higher VE rates on mobile are obviously because all of the worthless curiosity-clicks get dropped into the default editor. Approximately no one gets into the VE-bin unless they're an experienced editor deliberately swapping to VE. The desktop completion rates are also junk figures, dominated by the fact that a random percentage of wikis drop users into different defaults. Non-default usage is almost 100% experienced editors, with obviously higher completion rates.