Answered: Why can't I disable the Microsoft sign-in Assistant (Wlidsvc) service?

A customer asked me "Why can't I disable the Microsoft sign-in Assistant (Wlidsvc) service?", and I found a similar unanswered and locked question on Microsoft Answers here. 

I have an answer. The service is required for Windows Defender/MDE.  When it is disabled, its internal self-repair tools re-enable it.

Why Defender requires this service remains a mystery.