I agree. I am not into the partisan game, however. I try to think about what would help the nation overall. The difficulty is: it's incredibly easy to get painted as a partisan during moments like these, but moments like these are also the best times to real structural change to occur.
The FBI has had an open criminal investigation on Manafort since at least 2014. He was toast regardless of Mueller's involvement. Other than than, Mueller has come up with nothing significant.
Other than the "need", I agree with this version of events. I think if Rosenstein hadn't acted, Congress would have. There was outrage among both Democrats and Republicans at the firing of Comey.