I strongly disagree.
2007: Sacked 47 times. Had 32 touchdowns, 11 interceptions, and was second in the league in quarterback rating at 104.1, the highest of his career.
2008: Sacked 46 times. Poor statistical season, but only 15 interceptions compared to 23 in 2006. Also had four 4QC compared to only two in 2006.
2009: Sacked 50 times. Had a QB rating of over 100.
Where'd you get the fumble stat from? ESPN has the 2006 Steelers with 11 of 27 fumbles lost, 34 turnovers (Steelers defense had 20 interceptions and 9 fumbles recovered). By comparison, the 2008 team Ben threw 15 interceptions and the team had 26 fumbles with 9 lost, 24 turnovers (Steelers defense had 20 interceptions and 7 fumbles recovered).
I still believe Ben's injury played a major role in their failure in 2006. The Steelers started the season 1-6 and Roethlisberger had a 1:2 TD to INT to TD ratio (14 interceptions through 7 games)--this includes no touchdowns and seven interceptions through three weeks. Over the course of the season, the Steelers went 1-7 when he had more interceptions than touchdowns--with two of those losses being one possession games.
For comparison sake, the 2008 Steelers went 12-4 and won the title. He was sacked more times than in 2006, but he did a better job of protecting the ball. The Steelers were 2-3 (including a meaningless season finale win over the Browns) when he had more interceptions than touchdowns. His performance was particularly evident in one possession games, in which the Steelers went 6-2. In the six wins, 7 touchdowns and only 2 interceptions. In the two losses, 1 touchdown and 7 interceptions.
Roethlisberger is the most important player on the team and when he performs well, the team usually wins. When he's bad, he can be real bad and it's often too much to overcome. Given his numbers at the start of the 2006 season and that it was the worst of his career, you start to look outside factors that influenced his performance and two significant health issues can't be ignored.