It has been said many times that the Coordinator is not necessary because its confirmations will be rejected by other nodes if it acts maliciously.
It follows that if other nodes can tell if the Coordinator acts maliciously, then in fact they do not need the Coordinator.
According to The Transparency Compendium:
This does not mean that the IOTA ledger is currently in any way centralized, the network is 100% decentralized, every node verifies that the Coo is is not breaking consensus rules by creating iotas out of thin air or approving double-spendings
According to eukaryote31 (respected person within IOTA ecosystem):
What if the Coordinator started acting maliciously? Every node looks at the transactions it gets and only tells other nodes about transactions that are valid. The Coordinator is no exception, if the Coordinator starts issuing bad Milestones, nodes will just reject them.
According to iota.stackexchange high rated answer:
The Coordinator is no exception, if the Coordinator starts issuing bad Milestones, nodes will just reject them.
This begs the question, "what makes Coordinator Milestones a security measure if the other nodes are already doing this?"