While it is possible for objects in orbit to be knocked into a higher orbit, it's certainly not common. It basically requires a collision with another object in a highly elliptical orbit, this is not a kind of orbit we use very often.

Also, these low orbit constellations are simply nowhere near the majority of satellites, up in geostationary orbit. It's not realistic to imagine any debris from LEO ever reaching GSO, the distance between is just too vast. In general, Kessler syndrome would only extend downward from higher orbit, extending up to a higher orbit would be extremely unlikely.

Also, while astronauts could die, we keep enough emergency escape vehicles docked for the entire iss crew. NASA is full of smart people and they're generally risk adverse these days, I don't think anyone would die, but it would certainly be a shame to evacuate the iss.

Plus, the EU and China are understandably worried about Musk being the only game up there and want to deploy their own equivalent systems. So now there's not just one system of satellites threatening Kessler syndrome, but possibly three.

This is in fact a worrying situation. Not because I think Kesler syndrome is a realistic concern, but because there's only so much space in low earth orbit. I really don't like one company having a monopoly on low orbit communications, but having layers and layers of satellite constellations also seems like a dangerous situation.

Just roll out fiber everywhere like we have with electricity.

I'm all for that in theory, but whenever we dedicate funds to that cause... telecoms just walk away with it. If the US isn't interested in holding them accountable, I don't really see any reason to throw more money their way. That said, Starlink is doing fine, I see no reason to throw money at them either.