Good luck having all those well off people wanting to actually give the city of Los Angeles the money to leave, and I doubt LA would let that any part of it go without a battle.
In no way shape or form was the valley splitting from LA ever going to be a good thing for LA. Holmby Hills is way smaller but it still would leave a small ding on LA.
If Holmby Hills doesn't leave Los Angeles it'll just stay the same for them. It wont get better or worse. Same with the valley, leaving could have helped, there was a fight for the valley to leave, the valley stayed. Nothing changed in the valley. Although the valley splitting was more than just potholes. It was everything. Its apples and oranges here