I agree - that would be a wonderful feature for people wanting P2P with (or instead of) cloud storage. But there are already a few tools out there that offer functionality that Duplicati can use as a destination so it makes more sense to spend time adding backup features to Duplicati than re-implementing stuff that already exists.
That said, like Duplicati some of those tools are open source so there’s no reason somebody couldn’t take the two and combine them into a single instal file with a setup wizard to simplify configuration. 
Unfortunately I’m not that person - but if you post an “issue” for Duplicati at Github you can then put a Bounty (payment offering) on it. If other people are interested enough to put some money into having the same features added then somebody who DOES have the skills to do it might just make it happen! (the SIA bounty is a good example of how that can work).
“Easiest” is really in the eye of the bolder, er, implementor. I found Minio very easy to play with (testing only so far) and the Setting up self-hosted Minio on Windows 10 with automatically renewing SSL certificate How-To guide has been viewed over 100 times in the last 19 days.
Keep in mind, too, that your specific needs may vary from others - for example, I already have a good firewall and router set up with various port forwarding enabled which makes things like SFTP or Minio pretty easy to get to from the Internet.
But if you don’t have those sorts of things configured in your network yet, you may find ANY self-hosted destination harder to set up that guides might imply they are. Of course I’m assuming that’s what you want - for all I know you only have computers internal to your own network to worry about in which case port forwarding isn’t an issue at all…
That’s part of what makes CrashPlan and other cloud based services so convenient to use - THEY take care of the “server side” of things, so you only have to worry about the client. Eveb CrashPlan’s person-to-person backups still relied on their cloud servers to get the two two “persons” talking to each other…
If you decide to keep going with Duplicati and try to set up your own destination, let us know how it goes. If a guide helped you out, click that 
button for it, or if you had to do things slightly differently for your own setup feel free to reply to the guide and explain what worked for you - or even start your own How-To if nobody has yet documented exactly how you did it!