Investment businesses promote on evidence. By the time someone is made partner they have a long record of getting difficult things right, and the firm has every reason to be confident. That confidence is precisely why the transition is under-supported: the person has never needed help before.
But the criteria change on the day the title does. The work moves from producing answers to shaping how the firm reaches them, from personal credibility to institutional trust, and from a defined mandate to an ambiguous one. None of that is a step up in difficulty along the same axis. It is a different axis.
Most new partners get there in the end. The question worth asking is how much time, goodwill and senior retention the firm spends while they do.