I would argue that the people devoting the resources to coding and running the fork are the ones who can and will likely decide which accounts are the "cancer" that needs to be removed. Then we can decide if we agree and want to follow them on their new fork.
It's all about free association. I would hope that the community would agree about what is the "cancer," but there's always a risk of them not agreeing.
We all have choices to make here. Decide for yourself what is best for you.
Well, as you point out we can only associate with available mechanisms. For instance Steem. Is Steem perfect? No, not at all, and in fact we can see just how broken it is today. Will what's available be perfect? No. Let's just hope it's better.
But I think the question asked was valid. Who defines better? Hopefully the community does, and increasingly so going forward. Folks with the skills to make the platforms communities will choose between can do what @dantheman and @ned did, and enable golden parachutes like the founder's stake and dooming platforms, but if more choices arise, better platforms will win the communties.
We are seeing what golden parachutes do to communities, and it ain't pretty.