Isn't it equally likely the opposite - your comment presumes that Vanguard is using money from passive to prop up active, whereas it could also be that money from active is already being used to lower fees on passive?
> And what happens if active fails? Then passive would take the hit
What specific concern do you have in mind? Are you aware that the corporate structure of Vanguard is that it is the funds who own the company, not the other way around?