Is DMARC the end of email forwarding?

I think the problem you are coming accross is not the forwarding, but the DKIM alignment. Yes, SPF will Fail, but DKIM should Pass, but the DKIM Alignment might not pass. DMARC requires alignment to be strict or relaxed, based on your policy. You can read up more on alignments here: Identifier Alignments

There are testing tools like sending an email to [email protected] that will show the alignment on the results, but it's a paid feature.

Since this is a problem, there is a new specification in the works called "ARC" the intent of this is to solve this very problem, you can read about that here: ARC Spec

I asked Mailgun support about this problem and was not given any helpful answers. However, the solution is actually in their documentation:

Note If you forward messages to another email address, then you should disable click tracking, open tracking and unsubscribes, by editing your domain settings in the Control Panel. If these features are enabled, the content of each message is modified by Mailgun before forwarding, which invalidates the DKIM signature. If the message comes from a domain publishing a DMARC policy (like Yahoo! Mail), the message will be rejected as spam by the forwarding destination.

So, I turned off all of those features on my main domain which I use to receive mail and that solved the DMARC bouncing. If you want to use any of those features, you will need to setup a sub-domain for outgoing mail.