That itself is a huge problem and should be self evident.

If there is only one company with one team developing bitcoin, it's clearly not decentralized.

Every implementation is legitimate.

People choosing which implementation to run is people (noder runners/miners) voting what bitcoin should become.

I can't even understand what world view is behind an argument such as "the only legitimate implementation".

Open source means forks are legitimate at any time and even fundamentally important in a network like bitcoin. Without it, we could all just literally continue to use fiat anyway.

Why would you ever even say something like "the only legitimate implementation" ?!?