mailto:sage

If you were to try and send an email to sage, what would happen? Would it try and go to sage@sage.com or what?

No. It would go to the DNS root. I.e "sage@"

https://en.wikipedia.org/wiki/DNS_root_zone

We must consult the sage of the internet.

I bet the sage of the internet gets a lot of spam thanks to his/her/it's email address being all over world4ch.

Which root email would you want?
I'd like to get at@

I wonder how many emails sent to the DNS root zone are intended for other people. They just forgot to finish the email address or were expecting autocomplete to fill it in. Example: sending an email to alice@ and bob@ instead of alice@example.com and bob@example.com.
I wonder how much sensitive data they have that was never intended for them. They might even have GPG keys that weren't meant to be seen by anyone other than the intended recipient.