There's a lot of setup that goes into encouraging people to leave. Management works hard to make sure that people have greener pastures to go to. The people leaving don't see that side of it. If a partner encourages a client to hire Susie, and Susie leaves for the client, Susie might not ever know that a partner helped create that opportunity.

A friend of mine who used to work in management at Ernst & Young told me a bit about how this game is played. You need a strategy for getting people out that maintains the relationship, and offering backdoor bribes to lure people away is one way they do that.

I'm sure this happens, but the implied idea that McKinsey only keeps the best and only loses the people they intend to lose, is hilarious. Consulting sucks in many ways, even if you're consulting for McKinsey.

They don't have to be formally forced out, just realize that they're not likely to win the competition to be partner and move to other companies out of self-interest.

There isn't really any difference between what you said and what OP said, you don't have to be forced out to take the hint your job is a dead end.