Looks like you're right, only the primary broadcast stream appears to be must-carry and, even then, it's only as a legacy of the analog system. The digital must-carry rules appear to be quite fluid.
As for cord cutting, I'm glad it works for some but I just can't do it. I'm all by myself (for the moment) so there's nobody to answer to but me, and my sports, by definition, has to come via streaming (my favorite baseball team is remote), and I'm technologically savvy, so I should be a prime candidate. To me, it just seems like too big of a hassle for too little reward. I want to watch new episodes of shows as they run and not worry about which streaming service has what episodes, whether the boxes I have will play them, and whether there might be Internet congestion or outages at the streaming service. With my TiVo and some paid television (FiOS is preferred, of course), I don't have to check file formats or wonder about compatibility. "Oh, Suits is showing on USAHD. Let's record that."
If I went streaming-only, my status as a "lawbreaker" would skyrocket because I'd just say "screw it" and pirate everything, and I don't want to do that.