Teague, DeMarre Carrol, Sefolosha and Dennis Schroder weren't anything to write home about as young players. Heck, Carrol was a total scrub until he turned 27 and Kyle Korver could barely get on the court as a rookie.
I agree that the Celts have a lot of guys who could "pop" like the ones you mentioned if they found themselves in the right situation.
Having two All-Stars in the frontcourt has a tendency to allow that to happen. I think Teague was a bit of a late bloomer, too, though he always had the core prototype point guard skills to make a leap. Kyle Korver, as a pure shooter with below average athleticism, is a classic late bloomer, as well, though I don't think anybody expected him to shoot dang near 50% on a very high volume of three pointers for an entire season.
Teague has gotten better every year and he didn't have a massive jump this past season. He was a bit more efficient but that is about the only change from the prior year.
Korver has been one of the best shooters for awhile. Heck he shot 53.6% his last year in Utah.
Even Carroll who definitely shot the ball at a higher percentage last year, but otherwise was basically the same player from the prior year.
This notion that all of the Hawks all of a sudden became awesome is just silly. The 2 younger starters continually improved until the whole team came together last year. Good teams definitely help with efficiency, but that is pretty much what it was. Al Horford being healthy was the real difference between the 38 win team and the 60 win team. I think losing Carroll will hurt them a bit, but I don't expect this massive drop off either. The Hawks should still be a 50 win team as long as Horford, Millsap, and Teague are relatively healthy.
I just don't see that in Boston because I don't see as much progression or progression from a point that would dictate that guy could be a star (except for possibly Smart). I mean Olynyk looks better, but nothing about him says star. Sullinger hasn't improved much at all. He may already be at his peak. Crowder's efficiency actually got a great deal worse after he came over from Dallas, though that may be expected with the increased minutes. Bradley got worse last year and appears to be at his peak. Thomas is who he is. Smart is the only guy you can look at and say, well he may someday be as good as Horford, or he may not, but that day is certainly not today. Zeller is a backup center not much else. Johnson and Lee are nice players, but they aren't Horford and Millsap. They don't have the all around games or the versatility of those two.