Yes, you cannot attribute economic growth to Sen. Kennedy. That's why you have to look beyond the numbers to see his impact.
You're incorrect if you think that the peak of his influence was in his later career, when the glamour of the Kennedys had largely passed, and the House, Senate, and White House were controlled by Republicans. The peak of his influence was in the 60s-80s, but from that day onward he has been pushing health policy, and although he didn't get the ball to the top of the hill he's pushed it farther than anyone else.
To say that he was ineffective because the goal wasn't accomplished on his watch is like saying that Elizabeth Cady Stanton was not an effective suffragette because she died 20 years before women got the vote.
I'm evaluating his accomplishments with a bias to what he did in his later career, when he was presumably at the peak of his influence. And there the answer seems to be 'was skillfully co-opted by the Bush administration'.
I do agree that he was tireless in pushing health policy. I just don't think he was effective - but maybe we'll be pleasantly surprised by what comes out of the health reform meat grinder in a few months.