So what happens when you pay $100 more per person for a family of 2 for AC than for WS because it was on an Embraer with 2x2 seating and AC changes it to a 319? Does AC owe compensation?

What if you booked a flight because you really wanted a Dreamliner and they change it to a 333?

What if the 7M8 hadn't been grounded but the IFE just wasn't working that day, should you get a 15% cash refund?

People on FT are generally quite discerning about these things. We have our preferred aircraft types, maybe even our favourite seats on each type, etc. Most people haven't the slightest clue and don't make purchasing decisions on things like which airlines have seatback IFE. The number of people who made the type of purchasing decisions you're referring to is extremely small.

So what damages did these people actually suffer? Maybe slightly less legroom and no seatback IFE. But they got slightly wider seats, much nicer lavs, and available in-flight WiFi. The vast majority of passengers would have barely noticed, if at all. AC got them from A to B when they were supposed to.

AC offered people free changes to their flights, or full refunds. Apparently they also offered discount codes to at least some people. Those are pretty fair remedies to the damages caused by AC with the swap from 7M8 to rouge, in my view.

And a refund if 15% of the fare paid is pretty ridiculous. We all know AC doesn't charge 15% less just because something is rouge - it's not like the $300 premium for domestic/TB Signature Class vs narrowbody. Guaranteed that AC didn't decrease the fares on those routes when they switched the 7M8 to rouge.

So to answer your question, in your hypothetical situation, if you had a real problem with it, you should have complained to AC and gotten a change/refund/discount code/whatever at that time. If you're expecting a class action lawsuit to deliver you some compensation, yes, you're unreasonable.