> the sci‑fi premise of imprisoning ourselves behind a debris field feels less fictional

Yeah, no, the numbers don't work for this. The Kessler syndrome is bad, and worth avoiding, but you aren't trapped.

The trick is that you're not staying. Suppose a comms satellite in LEO would, as a result of a hypothetical cascade like this, be destroyed on average in six months but your space vehicle to somewhere else passes through the debris field in like 5 minutes. So your risk is like one in 50 000. That's not good but it wouldn't stop us from leaving.

The reason humans won't leave is more boring and less SF, there is nowhere to go. Nowhere else is anywhere close to habitable, this damp rock is where we were born and it's where we will die, we should take better care of it.