The Pareto principle says 80% of bitcoin would belong to 20% Bitcoin owners. This principle works for many areas and cases, I believe it continues to work rightly for Bitcoin distribution too especially if we only count unlost bitcoins.

https://bitinfocharts.com/top-100-richest-bitcoin-addresses.html
https://www.bitcoinmagazinepro.com/charts/hodl-waves/

If you use the second chart for HODL waves, and account for possible unlost bitcoins, you will see Pareto principle works.

No. Its decentralization still exists, with network hash rate distribution and with Bitcoin full node distribution.

https://buybitcoinworldwide.com/mining/by-country/
https://bitnodes.io/