So Web3 is built on blockchains which are decentralized and decentralization confers benefits such as censorship resistance, downtime resistance etc.

But blockchains require resources to operate. At great scale they will require great cost. What's to stop powerful actors from leveraging economies of scale and consolidating those resources, then ultimately using their control to manipulate the blockchains themselves?

This is exactly what happened with web servers. The web was initially very decentralized, due to economies of scale it consolidated and is hosted mostly in a few large datacenters now. The owners of those datacenters are by necessity very close partners with the government du jour, and now manipulate the content they host.