I don't think Brad Stevens is the reason the C's haven't won a championship since 2008 and why it seems that winning one this season is unlikely. I think there are 2 primary reasons the C's have been 2nd tier contenders.
1. Unlike the Nets, the Bucks, The Lakers, The Clippers... the Celtics do not have a top 5 (ish) player on their roster (yeah, Lakers have 2). Tatum is considered by some to be close to breaking into the top 10, but he's not quite there yet . NBA teams simply do not win titles without a top 5 (never mind without a top 10) player. It's happened (2004 Pistons; maybe 1989 and 1990 Pistons) but rarely.
2. The C's have lost 3 very productive players in the last 2 seasons. This means that before strengthening a nearly "ready" roster, they need to replace the lost assets first. Danny's had to do this with the loss of Al Horford and the loss of Kyrie and this year with the loss of Hayward.
So maybe Stevens doesn't use his timeouts all that well. Maybe the offense doesn't operate as some here think it should. But before Stevens gets blamed for not winning a championship, I'd look at not having top talent as the primary reason. Next, I'd look at the C's being unable to build upon their roster instead of replacing lost key assets.