What difference does it make if they can fork or participate? The important decisions are made by Vitalik and a small group of insiders. Do users get any input on anything actually important like the Ethereum fork that bailed out the DAO investors?

Protocol and client development spans hundreds of contributors and independent researchers. The discussions and development is public; just look at the EIPs, or even submit one yourself.

Users (non-devs) have agency through the software they choose to run. If mainnet was seriously affected by some attack, users could choose to migrate to a healthier network run by honest nodes. This agency happens with every hard fork: the DAO, but also the PoS merge.