Winning a Championship is hard. There are 30 teams in the NBA, so even if you assume that every team has an equal shot, the odds of going on a 15 year drought are about 60%. A 25 year drought? 42%.
A 7 year "drought" really is anything to complain about. Honestly any fanbase that's won a title in the last decade should lose the right to whine about anything. Never mind one that's won the most titles in NBA history.
Consider this for some additional perspective: 24 of the 30 teams in the league currently have double-digit title droughts. 21 have droughts longer than 20 years. 13 have never won the title at all.
When Celtics fans start getting all melodramatic about long title droughts, it just makes us look entitled and out of touch with reality. We've had great times and we'll have some lean times from now on, but that's part and parcel of being a fan.
As for the OP's question itself, I think it's more likely than not that we don't win a title in the next 15 years. But that's just the way the NBA works, and as a Celtics fan I'll be happy as long as we can put together a team that's in the conversation for several years within that period.