Unless you're Dan Marino you can't be called great without winning a Super Bowl in your career, and he at least reached a Super Bowl.
Romo would be in the hall of very good, along with other QBs like Steve McNair, Phillip Rivers, Boomer Esiason, Jim Kelly (reached 4 Super Bowls) and Warren Moon that didn't win a Super Bowl. Despite winning 2 I would put Eli in this category because he overachieved in the playoffs but was not great overall stat wise.
Then you have great QBs that won 1 like Aaron Rodgers, Brett Favre, you can already put Mahomes in this category, Drew Brees, Steve Young
Then the next tier 2 Super Bowls: Peyton Manning, Ben Roethlisberger, John Elway
Then Joe Montana is the 2nd greatest all time 4 Super Bowls, and Brady is the GOAT.
And I'm not saying it's all about Super Bowls and I realize it's a team sport, but the QB is considered the most important position not just in NFL but in all of sports and you have to do something in the playoffs more than just win 1 game to be memorable. The playoffs is what people remember. Peyton Manning and Aaron Rodgers are probably both better regular season QBs than Brady is, but Brady is so much better than them in the playoffs it's ridiculous which is why he's the GOAT.
So there's still time for Dak, but he will have to prove it in the playoffs. Until then he's proven to be slightly about average I'll give him.