The slippery slope argument doesn't really work in this case, especially since this is the first time in the gaming community that a reaction to something has been this strong. Instead, it shows that Bioware is indeed willing to listen to its fans when the moment really counts. To be honest, lots of developers say that they are willing, but how many of them would actually do that even when sitting upon mountains of cash?
The biggest point of all of this is that it's about sending a message. Bioware is well in their prerogative to decline to change the ending. Whether or not this risks negative implications in the future is up to Bioware entirely, especially since the buyers vote with their wallets. If this issue really is something very core to the buyer-seller relationship between Bioware and a fair amount of gamers, that's something to consider as well. Even mega-corporations eventually have to come down and say "we fucked up" to their consumers in the face of a big incident simply because their consumers is their life blood. This is not to say that everyone is forced to do this or that all business can or should do this, but I'm just not seeing why the gaming press seems to think that the gaming industry is somehow immune to this.
