<10K isn't the problem. It's when that 10K becomes much, much larger because of an unforeseen collision with an unrelated spacecraft/piece of space junk. It only takes one big collision to utterly litter that particular orbit, and thousands of slightly different orbits caused by the new trajectories of the various bits of the original spacecraft.

So whats your solution? Never put a bunch of satellites in space?

He's not necessarily saying he know's a solution, just that such a problem should be considered. Satellites do collide - https://en.wikipedia.org/wiki/2009_satellite_collision

The idea of satellite collisions and space junk is an actual concern https://en.wikipedia.org/wiki/Kessler_syndrome .

Sure a few washing machines in space isn't a big concern. Now imagine a few collide, hundreds of bits and bolts flying around earth, at unknown trajetories, at thousands of miles per hour.

There are a variety of potential mitigations. Legal requirements that satellites deorbit or park in a safe orbit before going dark, technology like https://en.wikipedia.org/wiki/E.Deorbit to capture and deorbit large chunks, better tracking of orbital debris, etc.

At any one time there are usually <10k airplanes in the sky,and they still hit each other sometimes despite going 23x slower than satellites

The vast majority of those collisions occur in areas where planes are forced to congregate (around airports, etc).

Isn't there high traffic where polar intersect? See: https://en.wikipedia.org/wiki/2009_satellite_collision.

To be fair, a lot of that is because of the fact that planes follow paths that are designed to keep collisions from happening, but aren't always adhered to.

There has been a collision between an Iridium satellite and another one before. It's a valid concern.

If they're travelling 7 km a second, maybe??