Change wmf-config/InitialiseSettings.php wmgVisualEditorSingleEditTabSecondaryEditor default from false to true. Individual wikis can still be set to false, if the local community so desires.
This will set the Single Edit Tab default for new users to wikitext editor.
EnWiki was set to false per community discussion (T132806). I stumbled across a Polish wiki request open unanimously requesting this change. (permalink.) I consider it very possible, and even likely, that other community requests have gone been overlooked and inadvertently ignored by the WMF.
Polish wiki should clearly be changed as a community request. If there is any question regarding changing the global default, I would be willing to do the work of opening discussions on several major wikis to determine if there is a genuine global consensus on the question. However I am fairly confident of the outcome, and it would be much simpler if we can agree on the expected result.