First of all, there's no rigid time allocation for a McKinsey interview where the interviewer is expected allocate time based on maximising your performance. Its also less about getting the "right" recommendation and more about the candidate's thought process leading to the "answer". If the quantitative section is a train wreck, which does happen, it's in the best interest of the candidate to just proceed so that the interviewer gets a good "maximised" read of other asepcts of the case interview. The Q&A at the end is purely a formality to create a positive experience for the candidate and so the interviewer will only move to Q&A once they have a sufficient read of the candidate's performance.