Just imagine that tomorrow all the big mining farms disappeared.  What effect would that have on btc?  After the difficulty had settled, absolutely none.  Correct me if im wrong but if there was a handful of compuers running the mining software then life would all carry on as normal.  btc is like the internet, you take a big chunk out and it carries on as normal.  One transaction would be verified every 10 min whether it was 100 miners or 100,000 miners.

Are bitmain, antminer and all the big mining firms and pools leeches on the system?

• As they are so big, the difficulty is higher and it takes more power to mine.
  
• All the power is concentrated in a few players.  Individual miners don't stand a chance.
• Chances are fees would be cheaper as well.