The Web PKI is hierarchical, but it isn't particularly centralized (other than Let's Encrypt increasingly eating everyone else's lunch, which is probably a good thing).
But in terms of actual failure points: if you're initiating a connection over HTTPS, then the only way an attacker can MITM you is by convincing a CA to incorrectly issue them a certificate for that domain. That's why Chrome and Safari monitor certificate transparency logs, and why website operations should also generally monitor the logs (to look for evidence of misissuance on their own domains).
