Imagine if tomorrow all the big mining farms disappeared. How would that affect bitcoin? Nothing, after the difficulty had settled. Correct me if I'm wrong, but life would continue as normal if there were just a few computers running the mining software. Bitcoin is like the internet; you can take a big chunk out and it will continue as usual. Regardless of the number of miners, one transaction would be verified every 10 minutes.

Bitmain, Antminer, and all the big mining companies and pools are leeches on the system?

Due to their size, they are more difficult to mine and require more power.
This power is concentrated among a few players. Individual miners have no chance.
It is likely that fees would be lower as well.