The idea Tycho would do something with his 'powers' as soon as his pool reaches 50% is just simply stupid.
Currently (in the current market) the guy makes 1000 - 1300$ PER DAY , that is 30k to 39k PER MONTH. In untaxed money even.
He would be stupid to do anything to destabilize the bitcoin market.
The idea that his pool is too large is not stupid, it is a valid concern. If you are worried about the security of bitcoin you should try to not put such a large area of trust in anyone's hands. While I agree it is not in Tycho's interest to do anything bad and really doubt he would..... an intrusion (attack) into his systems (online or physical) could effect the network.