At Blackstone, Mr. Schwarzman, for one, does not seem alarmed by SoftBank. When an analyst asked on a conference call last month about the threat posed by the new venture, the Blackstone chief noted that the Vision Fund was investing large sums in technology companies that are spending more cash than they are bringing in. In the private equity world, that counts as trash talk. Cash flow is its lifeblood; without it, companies cannot repay debt and pay dividends to their investors. For example, the Vision Fund has been making big bets on start-ups like Uber, WeWork (shared workplaces) and Wag (on-demand dog care) that are known for burning through cash, not generating piles of it.