re: I don’t quite understand how losing Bonham killed Led Zeppelin

Here's the official statement about their decision not to continue as Led Zeppelin:

"We wish it to be known that the loss of our dear friend and the deep sense of undivided harmony felt by ourselves and our manager, have led us to decide that we could not continue as we were."

In other words, they were so close that they felt Bonham could not be replaced. It had nothing to do with musical ability. It was like losing a family member and there was no way to replace that chemistry even if they wanted to.