Is Bitcoin a developer only currency and community? The Bitcoin repo on Github has 7 members. Ultimately, they decide what bug fixes and pull requests get merged. I don't understand how this is decentralized? Yes, it is open source, so anything they do merge can be seen by all, but how many of those all can really understand that code? Ultimately it will be the miners who will be responsible for pulling in new code - are we to say that all miners thoroughly review the source code and commit log before pulling in any changes? 

So this then means that the people behind miners are usually developers themselves? Which then basically means that developers are in charge of Bitcoin as a whole? If that's the case, are developers qualified enough to create and run a currency - do we not need economists as well?
Disclaimer: I'm one of those 7 people.

It is true that changes to this one particular code repository, and the releases of the Bitcoin Core software built from it are ultimately controlled by a few people, and many miners run this software.

However:


Bitcoin Core is not the only software interacting with the Bitcoin network. There are other node and wallet implementations.
You can run a modified version, or you can fork the code and maintain a branch yourself if there are features or changes you'd rather see.
Even with Bitcoin Core powering the majority of full nodes on the network, its developers can't directly change the core protocol rules. If a change was controversial, people can (and should) refuse to upgrade. Ultimately, it is the people deciding to run a Bitcoin node that indirectly set the network's requirements.
Miners do not set the rules. This is a common misconception; many people think the longest chain created by miners is the one that gets accepted by the network. This is not correct: it is the longest valid chain that wins. Every full node - including non-miners - verify the transactions in the chain and will simply ignore a chain with faults.