My belief is you probably need a core that has two guys in the top ten, or a MVP caliber player, or three guys in the top 10-25 range to have a championship level core.
Miami just went to the NBA finals with Jimmy Butler as their only top 25 player, and and I'd say he is only just top 25 (in the 20-25 range) too.
There is no way anybody else on that Miami team is top 25.
I know they got crushed by the Lakers, but still...they got further then other East teams who had more on-paper talent (including Philly /Embiid, Milwaukee / Giannis and Boston / Tatum + Kemba).
I think the crucial factor is less about star power and more about clutch power. You need guys who have the mentality and skill to make the big plays (be it shots, passes, rebounds or stops) in the clutch. You can be down by 5 or 7 in the last 3 minutes of a game and win - happens all the time. Late game execution is everything.
Granted, often times the guys who make those big clutch plays happen to also be big stars - guys like Jordan, Curry, Kobe, Pierce, KG, etc - however that's not always the case. There are a lot of big name star players out there who absolutely fall apart in the clutch (e.g. Embiid, Harden) which has a lot to do with why their teams seem to somehow always underperform come playoff time.
Likewise when Lebron won his title against the Warriors you could argue that it was Kyrie's clutch shot making (rather then Lebron's all round greatness) that won them that series.
I think Miami was able to get as far as they did because they have guys like Butler (who is as clutch as they come), Bam (who was also pretty big in when it mattered) and Herro (who made massive shots at crucial times).
Looking at this Celtics teams, I think we have quite a few guys on the roster who have that clutch gene. Smart clearly has it on both ends of the court (he's made quite a few big clutch shots), Tatum has it (he took over at the end of games frequently during the run) and from memory Kemba also has some impressive clutch shooting stats for his career. Guys like Tatum and Brown will make mistakes because they are young, but give them another year or two of experience and those mistakes will fade away - and that will change a lot.