Remember 2002-2003. That was Queiroz, as much as SAF, if you believe the rumors. In 2001-2002, the team was easily the most talented in the EPL. And it failed to bring home the title, conceding it to what SAF (and most United fans) still believe was a distinctly inferior Arsenal squad. Arsenal actually seemed like a better team in 2002-2003 before peaking in 2003-2004...but failed to win the title. In my mind, it's because of tactical improvement (and Rio, of course).
Queiroz is a good coach. He's a good starting point. I'm of the opinion that whoever takes over after SAF is taking a poisoned chalice. Heavy is the head that wears the crown, and all that. But Queroz has several real advantages: (1) he's not going to be compared to SAF as much as another British manager would and (2) nobody expects him to be a carbon copy. You bring in someone like O'leary, and everyone's instantly expecting him to be the "new SAF." That wouldn't be true with Queiroz.
I'd view him, ultimately, as a caretaker manager with a 2-3 year appointment. If everything goes well, then he gets the job on his own terms. If not, I doubt he'd be a disaster, as someone like Allardyce might.