It doesn't prevent spoofing, it doesn't say which IPs are authorized to send on behalf of a domain, it doesn't sign messages. Spoofed messages are still sendable.
Can you explain the benefit of only accepting email if Microsoft knows that you may or may not accept emails about scammers unsuccessfully spoofing your email domain? (Note, there's nothing you can do if they do tell you.)
Don't confuse DMARC with DKIM or SPF; DMARC doesn't actually do anything for email security.
"Oh, please, don't ignore the other two ways I told you to ignore unauthorized messages" shouldn't need a knob.
You get an email without a DKIM signature from me.
Is that because I do not use DKIM and do not sign my emails or because the message is forged and not from me at all?
You cannot tell. There is a single SPF record that you can look up to see if SPF is configured. It's just a TXT record on the domain. But there is no standard DKIM record. A DKIM signed email tells you what key identifier it used and you have to use that to look up the DKIM key at <key identifier>._domainkey.<sender domain>. When the email has no DKIM signature you cannot tell if that's because the sender domain does not use DKIM or if it does have a DKIM key record but it was not used, potentially because the email is forged.
Or do you think it's critical to support mail that doesn't have dkim?
Setting a DMARC policy that allows unsigned emails is easier than configuring DMARC signing including key management. It's just adding a DNS record. Although large senders should of course do DKIM and it's not that hard either
But I'm not arguing with you about whatever Outlook did.
DMARC doesn't introduce a new security mechanism by itself, but it fixed a few bad security flaws in SPF and DKIM. Without DMARC, SPF and DKIM do not add meaningful security.
DMARC updated and formalized how receivers should handle authorization and authentication of inbound email by adding the alignment requirement. Alignment ties the SPF and/or DKIM domain to the rfc5322.From domain. This also allows receivers to use domain reputation rather than IP-reputation.
SPF by itself has a bad design flaw that completely breaks any security that SPF was supposed to provide. By adding DMARC this security hole is fixed. (Although SPF is still horribly unreliable, it should be considered legacy at this point, don't rely on SPF).
DKIM benefits from DMARC alignment requirement too. DMARC ties the rfc5322.From domain to the DKIM signing domain, which makes DKIM practical for sender authorization, not just authentication.