Starlink satellites are designed to not cause this problem, and are on a low enough orbit that the problem would solve itself naturally in a few years anyway.
The time to deorbit is not linear. So this 500km orbit is fine while 700km orbit would get us ~25 year deorbit time. 800km over 80 years and so on.
See the scatterplot of debris for the Chinese collision test of 2000: https://en.wikipedia.org/wiki/Space_debris#/media/File:Gabba...
See this video for evolution of debris and altitude over time: https://www.youtube.com/watch?v=mQT5aMa_7iI
Also it was literally the worst possible collision being an intentional head on collision. Satellites accidentally hitting each other on orbit are very unlikely to hit each other head on as it is very uneconomical to put satellites in a reverse orbit that low (there is a reason why we do rocket launches towards east if possible)
The lowest point of the new orbit (perigee) is guaranteed to be no higher than the point of collision. Fragments after the collision have no further propulsion, so their new orbit must initially include the point of collision, and can only decay from there. This can also be seen on your scatterplot.
Moreover, the time to decay is most strongly influenced by the perigee, as the atmosphere is the strongest there. If a satellite on an low orbit (decay in decades) explodes, those fragments that "reach a higher orbit" will still (due to the low perigee) decay back to a near-circular orbit in decades. A collision can increase the lifetime for some of its fragments, but not by multiple orders of magnitude (unlike a circular higher orbit).
Your video link also nicely shows that fragments with a low perigee will decay quickly, no matter how high their apogee is.
There are many factors in whether this matters practically, so I'm not passing judgement on that. None of this is specific to Starlink.
You answered your own question.
More specifically, a Kessler syndrome is the problem that occurs when satellites collide and shatter into clouds of sand-sized particles, and then the sand-sized particles impact more satellites and shatter them, leading to a cascade that clears the whole orbit. Because small things deorbit faster than large things (square-cube scaling), this simply cannot happen at orbits that are low enough, at the Starlink deployment orbits if there was a catastrophic collision, most of the material would be out of orbit in a matter of days. At the even lower orbits of V2 constellation, most of the material will be out of orbit in a matter of hours.
The reason Kessler syndrome gets mentioned with Starlink is that the original system design had the early sats at 1100km and most of the constellation at ~800km. This would have been really bad, because that's about the worst possible orbit for collisions, as it's just high enough not to be swept clean by the atmosphere, but low enough that orbital velocities are very high and collisions are more likely. After the concerns were raised, SpaceX modified their design to be not dangerous.
Particles under 1cm in size don't cause Kessler because they're too small. Particles over 1cm in size don't cause Kessler because they are big enough to be tracked and actively avoided.
Everyone else is busy telling me I already answered it myself but you added the information I was asking for. :/
>SpaceX said that a large part of Starlink satellites are launched at a lower altitude of 550 km (340 mi) to achieve lower latency (versus 1,150 km (710 mi) as originally planned), and failed satellites or debris are thus expected to deorbit within five years even without propulsion, due to atmospheric drag.