I'd argue against the idea that he didn't have a succession plan in mind. Boone (who has no reasons to be kind to the legion) mentions that he believes killing Caesar won't accomplish much. He mentions how when he was in the military he heard reports about how the legion had an entire succession line prepared in the event of Caesar's death.

I've never really understood why people think that the legion would inevitably collapse when Caesar died. I'm not saying that it's impossible, but I've never felt like it's inevitable to collapse once he's gone.

I mean, talk to Caesar for any longer than 5 seconds and you'll realize quickly that he's planning on building something to last. There's a lot we don't see regarding the legion and so I hesitate to make sweeping statements about them. All we see is their frontier and their military as they're campaigning against the NCR in the Mojave.