Some of this depends on Fultz, who could go either way. But also, there are three things we need to define:
Future: Like, next year? 5 years? 10? In the next 0-5 years, I think Boston has a clear advantage. We have three all-stars (and one superstar) right NOW in Irving, Hayward and Horford. Over the next 3 years those guys should be remarkably effective together. In 3 years they'll be 29-34. In 5 years they'll be getting older - though it remains to be seen how they each age - but by then Tatum and Brown will be in their mid-20s and beginning their primes. 10 years from now, those two will be about 30-31. Frankly, I don't see the point in projecting that far out. Impossible to know what could happen between now and then.
Ceiling: A lot of the debate is based on the idea that Simmons and Embiid have the highest ceilings. OK, I want to deconstruct that. What are the absolute highest possible ceilings of each young stud? For Embiid it's gotta be Hakeem the Dream and for Simmons it could be Lebron. Tatum could be Durant, Brown might be Kawhi or even D-Wade. We're talking absolute best case scenario. Then the question is, what's the likelihood of each happening? Frankly, I think Tatum has a higher chance of turning into Durant than Simmons/Embiid do of reaching those lofty heights.
Draft/trades/FA: This is why it's so hard to project this stuff, but bear in mind Boston DOES still have a top-1 protected SAC pick next year (assuming LA does not convey) and that MEM pick coming up. I trust Ainge to draft well in the high lottery, if we have that chance. I also trust Ainge to trade and sign better than the average GM. Finally, I trust CBS to develop/maximize talent.
I'd say..... we're in good shape. Of all teams in the league, we're as likely to win a title next year as we are in 5 years.