And that's the point of TFA. More email senders have to set up DMARC, et al. When enough have set up DMARC, then it will be possible for your server to reject most spam.
DMARC isn't really a spam filtering system (nor are its components SPF and DKIM), just an "is this email from email@example.com actually from bar.com". If I'm getting spam from ilovespam.com it's not going to do much good.
Publishers on Steam can choose to have no DRM whatsoever on their game. Blame the publishers, not Steam.
Sendmail may be safely run set-user-id to root. -- Eric Allman, "Sendmail Installation Guide"