It totally depends on the situation. If the significant other here is the type of person who does not care one way or the other where their content comes from and just wants to watch Frozen, I see no issue with this hypothetical scenario. Though you would probably expect the family sysadmin to have already communicated the platform migration to the significant other before they try logging into the old platform.

Nobody forced you to do anything. You got told about a boycott and just started acting like a dishonest assclown. We had nothing to do with it.