Generally speaking, the higher the orbit the higher the chances of unrecoverable Kessler syndrome. For example if something went wrong way out in geosynchronous orbit and caused a chain of collisions, there's basically nothing we could do except clean it up manually — on its own it'd remain a mess for thousands of years.

For the lowest orbits, worst case scenario is just waiting 2-5 years for it to clear itself up.

Would it be a big problem elsewhere than in Geo ? All the Geo birds are basically in one plane and don't really move in repect to each other.

So I'm not sure you could really even trigger Kessler syndrome there & it would likely be quite easy to avoid that debris belt if you did.

Thankfully GSO is also 4300x as spatious, and we don't need as many satellites up there in comparison.

Not only is it crazy small, it's impossible. Kessler syndrome doesn't result in "space junk barrier" nonsense. That's from media and movies, not a possible reality.

> The probability of cascade-crash space-junk barrier is crazy small? Right?

Yes

Mostly because the low orbit means any junk would suffer atmospheric drag pretty quickly and decay.