Did Luffy not think Kin’emon would pull through?
The situation with Kinemon and the Scabbards were different. Luffy felt bad for not making it there sooner, knowing they stood absolutely no chance against Kaido, and if he had been even a moment later they'd all be dead.
Luffy has seen Zoro survive the impossible before. He has absolute trust that Zoro and even Law would pull through this. He thanked them for what they did for him, and said it was ok for them to go back downstairs.
Not to mention, remember what happened the last time Luffy let his emotions get in the way during a fight. Instant KOed. Now's not the time for him to go all crazy and start attacking Kaido haphazardly. The stakes are way to high, and he needs to keep his emotions in check.