The advantage of using a decentralized (global) verification network over a single certificate authority is generally durability and convenience of verification. A network is less vulnerable than a single point of failure. There is also the technical advantage of independent time-stamping, which has some security benefits that you can read about here.
It’s also important to note that most vendors provide hosted badges, not signed, for several reasons of convenience (more easily facilitating automated issuing triggers that may come from an LMS). However, badges that are simply hosted are not tamper-evident. Very easy to fool. Blockcerts do not have this problem.
Blockcerts originally started with the Open Badge schema, which temporarily led to some issuer dependencies as you have identified. This is obviously not ideal and work is underway for evolving to a DID-based architecture and Verifiable Claims schema created by the W3C. This will remove the final remnants of Issuer dependence for verification. The full roadmap is outlined here.
Thank you for this thoughtful comment. You have touched on an important aspect that is rapidly being addressed as Blockcerts continues to evolve with other existing standards.