You are right, the only decentralized thing here is the blockchain itself.
Could we have a way to get steemSQL decentralised ?
Then, decentralization as nothing to do with the topic of this post. Following your logic, one could argue that the Steemit.com website is centralized (and it is) and should be decentralized.
SteemSQL is a service I provide, but I do not have any monopoly on this kind of service. Anyone can do the same. Then comes the market that will chose the one that provides the best service at the best price.