and although Smith might only be the 20th best QB, I still call that successful.
At any given time, there are about 20 QBs who are solid starters, and a dozen other guys who are starting only because a team is giving them a chance. I would consider the former to be 'successful,' even if they haven't won much or gone to a pro bowl.
Current successful QBs are Romo, Rodgers, Brady, both Mannings, Brees, Luck, Roethlisberger, Wilson, Ryan, Rivers, Stafford, Flacco, Dalton, Cutler, Newton, Kaepernick, and Smith (who is right on the fringe of this list). Tannehill is probably on the verge of joining this list. If any of those guys were cut tomorrow, they would have no trouble finding another starting job.
Carson Palmer and RGIII are hard to evaluate because of injuries.
No college coach can claim more than one of the guys I have mentioned so far, and if you exclude RGII and Smith for not being good enough and Wilson for having played at two schools, you are left with just Harbaugh, Richt, and Patterson.
After the 19 guys listed above (not Palmer and RGIII) there are 13 starting spots left (Eagles, Redskins, Vikings, Bucs, Cardinals, Rams, Bills, Jets, Browns, Texans, Jaguars, Titans, and Raiders).
Those teams will try guys like Fitzpatrick, Sanchez, Manuel, Bridgewater, Foles, Stanton, Hoyer, Cousins, McCown, McCoy, Mettenberger, Locker, Glennon, Bortles, Manziel, Vick, G. Smith, Bradford, Schaub, Hill, Carr, Cassell, and Ponder. Any one of those guys (excepting the older ones, of course)could catch on to a spot and be playing for ten more years, or they may be out of football in two years.