If you do not see how it is bad, then you do not understand the importance in miner incentive and the model of decentralization. If three pools are responsible for 75% of mining hash, to me, this is centralizing the control of incentive for mining. The simple fact that companies like BitMain (Chinese) are making the software in their antmining products read-only is also sketchy. You buy hardware that remains in control of the largest mining pool? You cannot change default ssh passwords? Pretty sketchy.
Decentralization is termin related for bitcoin not for mining.
Bigger question is distribution, and it is not so good. Decentralization is guaranteed because no one controlling bitcoin as one entity such as banks or government, and this is accomplished using wallets, and transaction through blockchain..