This is the part I don't understand. Outside of the people who are riding this train because they enjoy seeing other people upset (and there have been quite a few I've come across during this debacle, self-admitting no less) they have no reason to contest the idea.
I can only assume it's because some people despise group content but continue to play an MMO for some reason. There's been a surprising amount of that coming out of the woodwork. Even then you'd think they should be fine with the concept provided they don't lose any of their solo content, so it just feels like it loops back around to arguing against it for the sake of keeping people unhappy.
Honestly it's not something I can fathom. You would think everyone would want to be on the same page for BLU to be the best version of itself that it can be that most people on both sides of the line could enjoy.