That sounds very frustrating. I wonder if their stance would be different if you had registered the power supply before the failure. Regardless, their stance doesn't seem right.
I still think that you should also contact Amazon, as Forgotten has recommended, and explain your grievance in a calm manner. Also, do you recall what method of payment that you used three years ago. My AMEX card doubles the warranty of my purchased items to twice to manufacturer warranty, up to two years I believe. Perhaps you used a method of payment that would cover you or perhaps your renters/home owner's policy covers this.
Also, when you spoke to MSI, how far have you escalated this matter? Or you speaking with the supervisor, his boss, etc? Or are you just calling the first tier in multiple departments. You could also skip MSI's chain of command and google and speak with a regional or district manager. My experience has been that they'll usually side with the customer in quick fashion, simply because their time is too valuable to spend a lot of time on such matters. If you google and get the name and office number, you certainly will speak to the gate keeper first. Just provide your name, contact information and why you're calling. When I do such things, I prepare for a few minutes before placing the call, by typing out what I'm trying to achieve and a rough estimate of what I'm doing to say.
The other option is to give this whole thing up for the sake of your own stress level. I suspect your frustration isn't about the money but about the principle and I can certainly understand that. Another option is for you to post your grievance on social media and/or on Amazon customer review site. If you do so, I would caution that you lower the tone of your words somewhat because you don't want to come off as a super bitter customer. There are additional options you have at your disposal, but I think they are more hassle than they are worth. Good luck man. I'm rooting for you.