Since Bitcoin is a community the developer dont just make changes to the codes without getting a consensus in the community particularly the stakeholders. But in essence developer can deploy codes but miners play a significant role for they are the one who have the computers/hardware to process the transactions. Do you think we have a monopoly in miner?
I think no , because Here the role of the developer team is to make the problem away from the network and make improvements time to time by the upgrade of the technology time to time ( here most common example is launching of segwit2x will happen by the developer team at 1 August ) .
But in case of the miners , You can't understand what the problems they are facing because the mining job are not much profitable for every place , so most of the people are making btc byy buy not by mining . Here the job of the miners is to solve the problems of the transaction whether the transaction is going to that particular person who want to receive or not .
Beside the miners there are something like node system which confirms the transaction .
So here it will.be more better to say that Bitcoin is not limited to a particular person of group of person .
In reality it depends upon many things and factor and supports ,so there is nothing monopoly in miner or in any other parts of this network system of blockchain .