Spot on post. Look at the NFL playoffs right now. How many teams that -- even prior to wildcard weekend this past week -- made the playoffs have a mobile QB? Zero.
Athletic QBs work in college, and in the pros it certainly doesn't hurt to have a QB with the ability to pick up a first down on their feet, like Elway or Young could, but the concept of having an elusive RB-like specimen at the QB position hasn't worked once, and NFL GMs have been trying it for 30+ years.
In the 1980s, Randall Cunningham was the first Michael Vick, and just like Mike, Cunningham went from HoF QB to bust to HoF QB again based solely on whether he posterized someone with a spin move or a juke. Show me a QB who rushes for 700 yards and completes 55% of his passes and I'll show you an NFL team that doesn't get past the first round of the playoffs. Not surprisingly, Cunninghams most successful season was probably '98 in Minnesota, when he completed over 60% of his passes but only ran for 150 yards. Once his knees were shot he had to use his arm and his head to stay employed, and he was a better QB for it.
As Ron Jaworski points out on the "Match-Up" studio show, you don't go from college to the NFL and get faster, more elusive, or more athletic. From the day you take your first shot from an NFL LB or DE, it's all downhill from there.