While I agree that it's wrong to pin too much of the blame on Romo, the argument about there only being 28 QB's who have won the SB doesn't really help Romo's case either. Even though there are some QB's who are considered greats that never won the SB, they are far ahead of Romo statistically, and most had at least some success in the playoffs.
Look at the guys you mentioned. Romo is 41,000 career yards, 271 TDs, 6 Pro Bowls, 3 All-Pros, a whole slew of playoff appearances & a Super Bowl appearance behind Marino; he is 29,000 career yards, 142 TDs, 6 Pro Bowls & several playoff appearances behind Moon; he is 23,000 career yards, 105 TD's, 3 Pro-Bowls & 2 All-Pros behind Fouts; he is 17,000 yards, 98 TD's, 1 Pro-Bowl & 1 All-Pro behind Esiason (who isn't actually even considered a "great" QB); and 15,000 career yards, 88 TD's, 2 Pro-Bowls, 1 All-Pro, a slew of playoff games and 4 SB appearances behind Kelly.
Romo will close the gap some, and may catch up with a couple of these players in a few statistical categories, but at almost 32 years old he isn't going to come close to the statistcal level of some of these players, and he is already well behind some of them in terms of playoff appearances and success. Another thing to remember is that the bar for greatness built on statistics is much higher now than in the past with the ever increasing emphasis on passing in the NFL. This year alone there were 10 QB's who threw for over 4,000 yards - including a rookie.
Don't get me wrong. I think Romo is a very good QB and given the right team and circumstances around him I think he is capable of winning a Super Bowl. But I have to say that his statistics alone are not likely to be enough for him to be considered "great". He is going to need more playoff appearances and more playoff success for that.