There's no one in "control" of bitcoin. The code is open source. Anyone can contribute. The nature of the system is consensus, so You could add to it. Here's the downside. The node operators have to agree to your alterations or they simply do not run your version and you get a hard fork or whats known as a altcoin(you ALSO can be a node operator no one is stopping you). If you look out there you will see that there is 10,000 alt coins.
to be a node operater/ independant dev that just 'follows' the current rules, yea anyone can make their own node thats wrote slightly different but still follows the main rules.
but wanting to contribute to an upgrade/change.. well thats not an "anyone can do that" thing
you hinted that you have to get wide co-operation with the other node operators or end up forking as an altcoin.
that alone shows its not as easy as you think.
secondly most of the other nodes are centralised to the "core" brand. and the devs of the core brand dont just let any old 'contributor' play around with a release candidate as they please. the top 6 devs have authority to accept or reject anything contributors put in. so there is a circle of 6 devs as management. and then ontop of them there is a single guy that does the compiling and verification of the release candidate.
so their actually is a central hierarchy that you wish to dismiss.. heck even that top guy(Wlad) admits he has too much authority and is a central point of failure for the project.
so if the 'consensus majority' are brand core, and a release of an update solely leans on the decision and signing authority of Wlad. then yes there is control.
Wlad doesnt want to abuse his control. but that does not mean its not there