Given the number of teams in the NBA and the way the league currently works, it's just not easy to win it all. There will be at least one team go 100 years without winning a title - it's not like the NFL where the deck gets easily re-shuffled. Unless something happens with the collective bargaining agreement to allow for more player movement, its just unrealistic for any team in the Celtics position to expect to win it all anytime soon without some luck.
And I agree, it could be a long rebuild. I'll go with 0.
But the as for who will be here when this team finally gets competitive again, that's a different story - I'll say Smart, Young, KO and maybe Crowder. Of course if depends on how you define 'competitive'. I define it as comfortably over .500.