I was thinking similarly. Not that it needed to rename it but that it was an obvious option available.
I think the choice to keep the main company as "Facebook" could be seen as hubris or overconfidence in the lasting trust in the name.
There's no question the company itself has been under attack for privacy, impact on social behavior, its profit and negligence in running ads that undermined integrity of the 2016 US Presidential election, and impact on the open web by making FB logins a universal user auth federation etc.
So creating a new super company name, like Alphabet, which sounds a lot like Altria (phillip morris) to me, that isn't directly tied to the other companies would have been a sensible direction.
Since he's the ultimate decider, it's hard for me to see this choice "Facebook's product: Facebook" as an extension of the personality and ego of the guy at the top.
Which is to say, the company does not believe the brand has been significantly undermined by various controversies. Or that it has but thinks it can recover and is fighting back in a way by keeping the name.
And admittedly, I suspect "Facebook" and its companies have a better idea of the moods of internet users than anyone else.