To be clear, you absolutely can still run your own email infrastructure, getting email delivered to you, filtering incoming spam, sending email (with DMARC signatures and other modern email practices), providing IMAP access, and even run your own webmail setup. You can even do this with all open source software. But the email environment you get this way is increasingly what I called an artisanal one. It’s cute, decent enough, and hand-crafted, but it doesn’t measure up in usability, features, and performance to the email infrastructure that is run by big providers. Your IMAP access might be as good as theirs, but things like your webmail, your spam filtering, and almost certainly your general security will not be as good as they have.
In short, if you run your own email infrastructure, it will not be up to the general quality you could get from outsourcing to big providers (they can’t really be called specialists). And you cannot fix this by trying harder, nor with the magical right choice of open source software, nor with the magical right choice of commercial software. Entirely “on premise” email is now an inferior thing for almost everyone.
I’ve always wanted to try and run my own email server, but I’d never run my main email address myself, since my income and interactions with the government depend on it. Still, it’d be a fun side project.