Not particularly knowledgable about this, but I was under the impression the front of the bus subsidizes the back. So if the front opts for more expensive supersonic flights then the back will have to increase in cost to make it profitable to fly subsonic flights at all.

Why? They already have price pressure due to some people not having enough money. If the people currently flying business class all move to SSTs, won't the airlines now have to charge more for coach seats fitted in their place because they were previously making much more profit per square area from business passengers?

that seems very unlikely. margins are already quite thin, and the airlines are making most of their money with the higher fare tiers. if you remove some of those customers, then its entirely possible that the base fare will increase in response.