McCown has been started 7 games in a row twice. The last time, his team went 6-7 with him on the field and 0-3 off. The only other time he had significant starting time was three years later in Oakland where he replaced a just 30-year-old Daunte Culpepper, who didn't fare any better than McCown did.
This is the just the second time he's consistently started, the third our fourth time in a decade that he's had appreciable playing time, and the first time he's had a decent supporting cast around him. In Arizona, he had two rookie wideouts that turned out great -- Boldin and Fitzgerald. Arizona replaced McCown with Kurt Warner, who went 2-8.
While McCown is almost surely not this good, it is not impossible that he is much, much better than the McCown of the past that played sporadically for horribad football teams.