I mean, that POS centralization is more difficult. First of all, it's expenses: electricily and hardware, that becomes broken and which you can't convert to money without losses, so you really need to withdraw profits. And the second, it's electricity. It's very difficult to accumulate electric power in one hands, because of techinical problems. Could you imagine, that one miner (not pool) owns 20% of hashing power in the world like otoh? At least, he will need to have it's own atomic electric station, and maybe not one. And what about masternodes? There are no expenses to own masternode, except hosting, which you can optimize strongly. You invested money, you withdrawn money. That's all. But if you will recapitalize masternodes, you will own new masternodes much more faster, than other holders and in perspective your share of total masternode profit will grow in %. For example, Otoh owns 642 MNs and other 642 owners have one MN each. With time, Otoh will enter new masternodes only from profit and grow his percentage and other owners will wait a lot, while their profit reach 1000 DASH. And as more time, more MNs and % will have Otoh and 1000 DASH goal for others will be more futher. With mining it's more difficult, because your hardware may obsolete and you can't simply grow your electric power, because it has limits. With MNs you have deal only with money. In future, somebody with most masternodes can accumulate most new generated money without additional investments.
This is always going to be the case with anything having an open, free, market. Otoh bought his coins on the market like anyone else can. Understandably, people with more resources have a clear advantage, no different than any other thing in life. If people weren't willing to sell their coins cheap, Otoh's position wouldn't have grown as large as it is presently. Further, it's a two way street--there isn't the liquidity for him to just dump all those coins on the market if he wanted out. Yes, he's able to compound his coins faster than people with smaller holdings--there is no way around it. He also took a risk with his capital to get involved. You bitching about something that happens all the time in life is silly.