I was talking to a friend who is in the Telecom industry in Japan, and apparently this sort of arrangement is not legal there. EU is generally wary of such arrangements as well. So this is a merger whose product synergies would not have been possible in other jurisdictions.
In recent years I recall advertisers being skeptical about the quality of eyeballs on Yahoo!'s platform. The pitch to the same advertisers already seems more compelling, though the premise does make me feel uneasy.
And I imagine Mayer will be getting her full 9 figure severance package. So much for rewarding success and having interests aligned.