Since the mod itself is likely never going to be ported, and the original project has such wonky licensing restrictions, what is the end goal of keeping the assets available?

I don't mean this in a "why even bother" way, I genuinely want to understand the purpose of doing this.

There is no goal, other than a different appearance, at least for some. But I use them to add my own content to my games. I play a unique Facrtorio that no one else plays, with mods that I will likely never publish. I expect there are others that are capable of modding their own content too, and like me, have only basic graphics capabilities. Having access to the IR3 assets can go a long way to making your own content look nice.