Well, of course. It’s only fair. You can’t change a rule then go back and hold prior behaviors accountable according to the new rule. It’s just common decency.
It's also not about suits between individuals. It's employees seeking damages from Google, so there's no third party outside Google and the plaintiff who could be harmed by the switch. And the plaintiff shouldn't be harmed, because removing the requirement for arbitration wouldn't deny it to anyone who wanted it.
As far as "holding Google accountable for prior events under a new rule", that's pretty much what people were asking them to agree to in the first place. But even worse, Google's statement says all future claims - even about past events - will operate under the new rule. So Google is already going to be held accountable for past events under this rule, and this is just disadvantaging the people who already opened claims.