See and avoid doesn't work so well when you're in the clouds.
No, but commercial aircraft in high traffic areas tend to have TCAS and similar to alert them to traffic, and if on a proper clearance won't run into anyone anyway.
Also, you might not see an aircraft coming at you until it's too late.
Like I said, they are humans in the cockpit, and their failure to be perfect at see-and-avoid doesn't mean ATC is the only person keeping them apart.
So, yeah, the OP was right.
No, he was wrong. The pilots are also there to keep planes from running into each other. If you are going to discount their presence because they are imperfect at it and think only ATC has that job, then you better discount ATC as well because they are not perfect, either.
In many (most?) situations, controllers are the only people stopping planes from running into each other.
Bullshit.