Kessler syndrome is incredibly overrated.
It's completely incapable of "permanently blocking access to space". What it's capable of is "shit up specific orbit groups so that you can't loiter in them for years unless you accept a significant collision risk".
Notably, the low end of LEO is exempt, because the atmosphere just eats space debris there. And things like missions to Moon or Mars are largely unaffected - because they have no reason to spend years in affected orbits.
LEO is indeed exempt (which is a great thing given that it's getting quite crowded up there). But we could easily break geostationary orbits from being viable, as they don't decay from atmospheric drag.
In the ISS decommission report that evaluated different retirement plans [1] for the ISS, the suggestion to park the ISS in a higher orbit was evaluated but dismissed because raising its orbit out of LEO would increase collision risk to >4 years lifetime, and raising it further requires too much fuel.
ISS is currenrly in the higher end of LEO, meaning most debri decay away slowly. But higher orbits are already hazardous, and our space development is still very small-scale in those orbits. "loiter around for years" is already at 5 years. And that's with a relatively small amount of development and short history. If we want to do space in anther 100 year without inch-thick steel armour on our rockets that leave earth, we need some regulation around this.
1. https://www.nasa.gov/faqs-the-international-space-station-tr...