I found this piece in an official Tableau Online documentation (see Notes section in the document by the following link):
Tableau recommends that you dedicate a site administrator account that is always configured for Tableau authentication. In the event of an issue with your Identity Provider, a dedicated Tableau account ensures that you always have access to your site.
So, looks like that it is recommended to always have one backup site administrator with Tableau authentication method.
We can consider this question as closed if there're really no way to revert access to Tableau site when Identity provider is broken.
Will mark as Closed
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