I admit, I had some concerns about that as well. But our UX team and PMs did user surveys they believed to be broadly representative and found that enough sites

I admit, I had some concerns about that as well. But our UX team and PMs did user surveys they believed to be broadly representative and found that enough sites are doing split logins now that our users didn't have any trouble with the change. This might be a factor of having been done just this year. A year back, the decision might have broken the other way.