Please REVERT or FIX immediately.
@Jdforrester-WMF you explicitly assured me that the WMF was not going to try to slip in a Visual Editor default as part of the Single Edit Tab deployment.
I just created a new account. I went to a random article. I was given only one edit tab. Clicking editt freezes up my browser for a random timespan while Visual editor is forced to load first. I was assured that Wikitext was the "Primary editor" and that the "Primary editor" would load first. Then a popup appears with two options: "Continue" in the current editor (fine), and a "Switch" option to initiate a load of the alternate editor. The "Switch" option will need a text-tweak to accurately indicate that it's switching to Visual.
(Oops, sorry. I did not mean to replace the description. I intended to reopen and write a reply. I'm trying to restore the original description, but I have not yet found a revert option or a copy of the original text.)