First, it assumes that the pool of diverse candidates has already been exhausted. Anecdotally I have not seen evidence of that. If it was the problem you claim, we would expect female engineers to make much higher salaries than their male counterparts. There is no evidence that is happening. This will eventually be a problem once every company tries to be more diverse than the pool of candidates, but right now there does not seem to be enough companies prioritizing diversity to an extent that this is an issue.
Secondly one of the causes of the pipeline problem is not having enough visibly successful diverse people in the industry. The more diverse candidates that are put in positions to succeed, the more visible they become to the next generation, and the more likely the next generation believes that this career can be a path they can successfully pursue.