Saying that 15 witnesses control Steem is not exactly accurate. You need a consensus of 2/3 of the witnesses (if I recall correctly) to determine which is the longest chain. That's about 15 block producers. For a hardfork you need 17 to reach consensus if I am not mistaken. However, the rewards pool is allocated based on the stake-weighted voting of the SP holders. That is a different layer of decentralization (or lack there of).
However if we only measure the number of block producers then a blockchain like Steem is"more" decentralized than bitcoin (for example) which has 8 mining pools with 84% of the hashing power. But that is another story.
8 or 17 still about the same in terms of decentralization in my book. I'd like to see account based witness voting, that would be much more decentralized than we have now, which is a plutocracy. But that'll never happen because you would never get a supermajority of witnesses to vote themselves out their own positions to make it democratic.
Fore account based voting we would need to have some sort of proof of identity or uniqueness of an account to prevent sybil attacks. Otherwise someone could easily create a multitude of accounts and replace the witnesses with possible nefarious intentions. Stake weighted voting prevents that.
Ultimately it is a user with millions of Steem power who controls who gets to be in the top 20.