From a technical point of view, they are not centralized. Gmail is a massively decentralized system in itself, a feature partially enabled by emails flexibility. Even if gmail would go down, (other) emails would still continue flowing. And when gmail would come back online, most messages would find their destination due the store-and-forward nature of email.
The fact that even you mention several providers shows that federation does work.
Currently the major pain point in the architecture of email arises when people bind their online identities to singular email providers, even when email has nice facilities of not doing so by having a domain independent from the mail provider and setting appropriate MX records.
From a legal perspective, GMail is operated by one corporation which can have a single warrant issued, and be forced to reveal all of your emails, no matter how many servers your account resides upon.
Even if you do end-to-end encryption, they're still going to see who you're sending email to.
With a more decentralized system that for instance does onion routing it might be possible to radically improve privacy.
The fact that even you mention several providers shows that federation does work.
Currently the major pain point in the architecture of email arises when people bind their online identities to singular email providers, even when email has nice facilities of not doing so by having a domain independent from the mail provider and setting appropriate MX records.