Collusion isn't just possible. It's already been going on for at least two years now. As was said by one pool operator (paraphrasing), "the only people mining are 12 pools".

Bitcoin isn't about distrust. It's about decentralized trust as that is the primary function of the blockchain. Bitcoin as an ecosystem hasn't been about distrust, but decentralized trust, and it has long been moving away from decentralization, which is fundamentally problematic.