Why is the current time and redundancy requirement insufficient? They had plenty of time and opportunities to look at it, and they succeeded with time to spare. What is the scenario where they would run out of time, and why couldn't they land assuming that the landing gear wasn't deployed?

You'd need potentially an unrealistic number of cameras. Even if you decide on a case by case basis, you still have to weigh the risk that every component adds, and with a finite amount of money to spend on risk reduction you want to get the most bang for your buck.