What is significant about this group is that while Tester and Heitkamp and Manchin are facing tough reelection fights in states that went for Trump, Durbin, Blumenthal, and Coons have seats for life and they have strong credibility with the Democrat base. If they aren’t pushing filibuster then, more likely than not, it isn’t going to happen.
Schumer has pretty much locked himself into pushing a filibuster and daring McConnell to get rid of the filibuster. This is going to strike directly at his ability to lead his caucus in the coming fights.
Not only that, but Durbin's position as minority whip speaks loudly here as well.