I see. Thanks for pinpointing the source of confusion. When Satoshi wrote that, no other viable method of securing a blockchain had been invented yet. His comment in the Bitcoin code is not a dogmatic definition to last forever.
Proof of work is not required for a secure blockchain. PoS, DPoS, and other methods have since been developed that are able to securely produce a chain of blocks linked together with cryptographic hash references to previous blocks.
No. PoS and DPoS are regressions to Merkle Trees, that's the whole point. They are community-administered solutions (at best, if at all) to a problem that Satoshi solved technologically.
PoS at most is inelegant Proof-of-Work. Do your homework and understand why. Only then come back to further discuss.