Manageable isn't just from Amazon's side, it is also an argument from the side of the plaintiffs. e.g. "This class is too diverse. It will be like herding 300 million cats, which decreases the likelihood of arriving at a suitable settlement. Better to split this into multiple classes with more specific grievances".

Everyone benefits from the class being manageable (except maybe the plaintiffs' attorneys, who just want the biggest class they can possibly find).

"decreases the likelihood"?

What's the difference between a class of 20M and one of 200M? 99.98% of them aren't being surveyed for specific grievances, they're ticking a box on a letter or on a website. Let's not pretend that they're all getting surveyed for their desired outcome.