Why cold-launch missiles still use vertical cells

Should cold-launch missile cells be angled so an ejected round cannot fall back onto the deck?

Generally no; properly functioning cold ejection clears the ship, while an angled cell adds complications and does not cure a major launch failure.

A cold-launch system ejects the missile before its motor takes over, which creates an understandable concern about a failed round falling back. Navies already use cold ejection successfully, including in systems such as Sea Ceptor. Once the missile has been expelled with the required momentum, it should continue clear of the ship; a round returning to the deck means several failures have already occurred. Tilting every cell would add the structural and exhaust complications discussed for box launchers without removing the deeper reliability problem.