Yes there is no need of knowing how much someone else owns but there is definitely a need for who stores your bitcoin. In as much as bitcoin isn’t all about complete anonymity but rather it’s pseudonymous and can be used anyhow, not having self custody of your bitcoin seriously goes against most of its principles like total freedom which bitcoin was built upon, handing over bitcoin to institutions is no different from money in the banks you don’t get to use your money every time and any how you like because there is restrictions most at times.

But also using custodial storage doesn’t actually affects bitcoin decentralization because bitcoin nodes as long as it is been run by many will actually keep the network securely decentralized. But is there anyone that will be handing over his bitcoin to centralized institutions and still be running nodes, I don’t think so, node runners are self custodians and that’s why it is heavily advised to go self custody to encourage more node runners