Running the same software is not a problem, as long as the consensus rules that this software are decentralized.
Agreed. If we couldn't trust Bitcoin Core, we'd have bigger problems to worry about. And even if the developers ever go evil and add bad stuff, we'll just stick to the older version and go from there.
The problem is not just trust. If everyone is running the same software then there is a risk of a serious bug that could take down the entire network. With multiple implementations, that risk is limited.