I agree with you, but I do think it's fallacious to zero in on Salesforce in the example. That wasn't (likely) intended as a defense of Salesforce, it was just an example showing that there is a long historical usage of third party cookies that aren't purely for ads and tracking. It's impossible to predict how many things might break for people. For me (a uMatrix user and Firefox user) it will be zero. But for people who work and compute inside a corporate office using internal software that was "optimized for IE 6" but they still need to do their job, and the customers they serve (I have no doubt there would be plenty of government offices that will have to tell citizens who need government services to go pound sand), it will be highly disruptive. Dismissing all of those people and use cases with "who cares about Salesforce" is myopic and (for anyone with decision making ability) deeply irresponsible.

That said, this has been on the table now for years so it's time for the operators of these old apps to feel the heat. The Times They Are A-Changin'