Collisions aren't theoretical, near misses are so common that there's an entire department at NASA dedicated to detecting them and warning satellite owners to adjust course, I know because we were contacted about a possible collision involving our cubesat. Prior to megaconstellations being deployed if humanity stopped adjusting satellite orbits there would be a collision within a month, now there would be a collision within 5 days. It's only a matter of time until both satellites on a collision course don't have the ability to adjust course (engine failure or no propulsion/fuel/comms). In the event of a Carrington-style solar flare there's a good chance a decent percentage of satellites would be knocked out, making this hypothetical into a reality. Further, we can only currently track objects down to about 10cm, but NASA estimates suggest about 500,000 objects exist between 1-10cm in size in LEO.