What's centralized about what i said? You think one person having 10% of a network is centralization?
If you really believe that 10% is menial, give me 10% of your cryptos, then tell me how decentralized you feel in the morning.

One person owning x% of the Masternodes is a very different thing from one person owning x% of the hashpower. Probability of attack success does not scale linearly with attacker numbers when it comes to Masternodes.
Masternodes are selected each block/transaction effectively randomly, in an n-of-m fashion. Do the math on the chances of 10% of Masternodes belonging to one person being compromised enabling anyone to trace a transaction through for example 8 rounds of Darksend:
0.1^8 = 0.00000001.
Good luck with that.
At 51% compromised Masternodes it rises to a mighty 4.58% chance of success, vs a 51% hashpower attack which has a 100% chance of success.
If you're not getting that Masternodes are fucking awesome, I suggest laying off whatever shit you're smoking and injecting some steroids directly into your brain. Might take you a few stabs to locate it...

It is supposed to be random but if you will read instant tx whitepaper you will see that they wanted to use some very non random method of selecting masternodes which is easier to attack although whitepaper assumes (incorrectly) that it is random as you describe it. The actual method of selecting masternodes in source code is different from the one described in whitepaper, they are subtracting points in 4D space and then make some arbitrary bit shifts, it is hard to tell how secure it is. The correct method of selecting masternodes should be random which is easy to accomplish using good hash function such as SHA-2 or X11, no need to invent new algorithms, standard hashes are known to be good and secure (and will produce uniform distribution which is important here).