"Would you want an group that doesn't add value to steem to benefit from 20-50% of the entire steem reward pool?" As I said before, if they'd have this big of a share, it would be in their best interest to provide value. Pulling out takes time and if that would be to happen, people would eventually scoop up enough cheap Steem to make it rise again. 
Or do you think they would blatantly use the system, making everyone else power down as well, hurting their profits immensely? 

It's good to be thinking about these scenarios. I'll think about it, but if some major player would begin to try and get all liquid Steem, let's say 50 million out of 250 million total supply. This would drive up price insanely as in your scenario they would need to power it all up. As they drive the price up, those who have the 200 million can start selling as prices go lunatic (imagine minimal liquid Steem) and buy back lower. 

These are just my initial thoughts.