I would say that the vision fund was predicated on a audacious idea that was either pure genius or absolutely idiotic. The idea is as follows, instead of investing our huge pile of money into a bunch of different companies, which takes a lot of legwork, let's just fund a relatively small amount and give them a bunch of money. We will give them so much money that it's not possible for them to do anything except totally crush the competition and control the entire market.
The problem with the idea, and the eventual reason it didn't work, was that though it was largest VC fund ever, 100bn isn't actually that much in the scheme of things. There's so much money sloshing around the system looking for the place where it will be treated best. So, in essence, what Softbank ended up doing was bidding up the price of the entire industry, creating massive inflation. If Softbank gives a shi-tton of money , and another VC gives a shit-ton to a competitor, they've both just wasted a shit-ton of money.
I actually don't think Masa is the compete idiotic everyone makes him out to be. He has a very high tolerance for risk, but that doesn't make him irrational. The Vision Fund might not be doing that well, but Softbank's stock is looking pretty good at the moment.
