As I understand it, Rosenstein has a reputation of being some even player guy on Capital Hill. You could be right that he couldn't take any pressure so he even things out in his mind by appointing Mueller.
And Comey had that same reputation and it was mentioned often up until his controversial moves re: Hillary during the campaign. Mueller has that reputation. I think these bipartisan reputations inside the Beltway have a big quotient of you slap my back and I'll slap yours, a lot of disingenuous, mutual butt kissing.
But it does look like the Rosenstein hire was a very bad move as there was no great urgency to consider a special counsel when Rosenstein suddenly announced he'd hire Mueller.