At some point, a hard fork will be required for Ethereum that the community will not want because they've got vested interests that differ from Ethereum's developers.
Once the transition is made to proof-of-stake, the incentives of its "miners" will be more closely aligned to those of the community.
The existing miners can't prevent that transition. They're also using general-purpose hardware that can be repointed to other chains or sold to gamers, so they don't have a strong incentive to fight it anyway.
If the community ever loses confidence in the foundation's leadership it can transition to another arrangement.
But I'd only claim it's more resistant, not immune.
It sounds like what you're saying is "yes, big players will emerge but something will keep them in check." What is that thing?