I think it's a case of a guy who just wasn't ready and was promoted too soon. He went from a position coach one season to a DC the next season and to a HC the next. He would have been well served to have gotten another season or two of seasoning under McVay instead of running off to take a HC job.
But more than that, he should have held out for a job with a better organization/ownership. Dean Spanos is one of the worst owners in the NFL and largely a POS as a human being as well. These guys like Staley want the top job so bad they'll move too fast and take anything that comes their way. This result has seemed inevitable for a while now.