In what sense? I think in practice there are significantly fewer widely used email service providers than there are web service providers. If you threw a rock at a crowd of people, you'd probably hit someone with a Gmail or Outlook-managed inbox.
Signal is notably not proprietary. And email is de-facto owned by a small handful of service providers.
Telling Joe Shmoe that he should run his own email infrastructure instead of using literally anything actually built for E2EE is an ideological argument, not one grounded in Joe’s message security expectations.
See parallel response. Open source is not the same as an open interoperable standard.
> And email is de-facto owned by a small handful of service providers.
No, not really. Yes there are large providers who manage a lot of people, but it is not owned by anyone.
> Telling Joe Shmoe that he should run his own email infrastructure
That's not necessary either. Joe can get his email from any of thousands of providers ranging from large to tiny if he doesn't want to run it. Service can also be delegated in various ways depending on comfort and convenience. For instance, one mixed setup is to manage receiving by one provider (which could be oneself, to guarantee you can't get locked out) and delegating sending to a different provider (self or others).
It's also easy to delegate to a tech-savvy friend or family member. I run email for my own domains but also for most family members and a few consulting businesses in our circle.
This is the power of open standards codified in RFCs. It is what the Internet was meant to be. Walled gardens was never part of the plan.
The Delta Chat ecosystem runs on systems that only accept encrypted emails. You access it through a Delta Chat client, you don't use a Gmail or Outlook-managed inbox.
That's already a lot more decentralized than most web services we use on a daily basis