How decentralized are Bitcoin and Ethereum really?

A study of the state of crypto networks (including Bitcoin and Ethereum) revealed:

• Bitcoin Underutilizes Its Network
• Ethereum is Better Distributed Than Bitcoin
• Neither Are All That Decentralized
• Ethereum Wastes Mining Effort That Can Be Put To Better Use
• Ethereum Exhibits Better Variance in Fairness, Favoring Small Miners

Kinda crazy that mining for the top 2 coins is actually very centralized. The top 4 minters control more than 50% of the hash rate and "the entire blockchain for both systems is determined by fewer than 20 mining entities".

Here's the link to the study: http://hackingdistributed.com/2018/01/15/decentralization-bitcoin-ethereum/