Dead satellites do add a much larger risk than satellites that can be steered, sure. If we stopped steering all our satellites right now, I believe it’d only take a few days before a collision occurred.

However, every satellite in orbit adds to the risk, especially if a chain reaction starts happening and it becomes very hard to avoid the shrapnel flying around. Or if a once-in-a-century-type solar flare takes out a bunch of satellites.

Edit: Basically, the best way to prevent Kessler Syndrome from occurring, is to keep the number of satellites in orbit below the threshold where it could occur.