It is generally not required that the candidate "cracks" the case. This also varies widely by case, and interviewer. Some interviewers like to give very long, difficult cases, where completely solving it is very, very difficult, and the vast majority of candidates (including those who get offers) won't solve - this was the case with one of the partners I worked with at Bain.
Other interviewers will give more simple cases, and in this instance it is more of an expectation that the candidate with "solve" the case. In these cases, clients are more differentiated through the additional "gold dust" they provided in the case - for example, not just having a structured recommendation at the end, but going deeper by proposing next steps.
Finally, I am not sure what Vlad is basing his statement about "bain expects you to crack the case in 70% of cases". This is definitely not true, and I don't believe there is any specific expectation about candidates being able to come up with a specific answer.
Hope that helps!