Isn't that what is slowly happening? My understanding is that it's becoming harder and harder to send email to a Gmail address of you are not yourself using Gmail (without being tagged as spam/junk I mean)
I host my mails and to my surprise, gmail has been one provider which hasn't caused me any trouble so far in 2 years, with Yahoo.
I have proper configuration for dkim, SPF and dmarc. Even when my mails were somewhat misconfigured it worked with gmail.
When a recipient has a gmail or a Yahoo address, I am quite confident that my mails will make it into their inbox correctly.
Not that I'm glad that most people I email with use gmail, since I'm precisely hosting my mails to escape Google (well, this is one of the reasons).
I cannot say that for Microsoft.
After a long discussion with them, I think I'm not systematically landing in my recipients' spam folder anymore, but I'm still not very confident.
Microsoft, please fix your filters! If I already exchanged with somebody in a two-way direction several times, my mails should not land in their spam folder anymore, especially if they explicitly marked me as not spam (even more if they added me to their contact list), and if you observed only legitimate mails for months or even years from my IP/domain, my mails should not be treated as spam by default even for new recipients, right?
Good and reasonable filters are important for internet neutrality, where small providers can exist.
It's still federated, its just harder to federate your own instance. It is still totally doable you just have to jump through the hoops of setting up SPF, DKIM, and DMARC.
