I do agree with you that it's great to support other engines! In fact, our Humble Bundle came with a game engine: 001 Game Creator, so it was definitely not RPG Maker specific. None of the assets from me or the other participating creators are exclusive to RPG Maker, they can be used in any game engine. This is particularly so for the music... they're just music files, and work in anything that supports .ogg and .m4a files. I have a friend who is making a pseudo-2D RPG in Unity that is using assets from that Humble Bundle (and others).
My primary market has been RPG Maker users for the last six years, and they generate the vast majority of my sales (mostly as official DLC for the engine on Steam). I've had listings on the Unity store before, and in general it wasn't a big driver of sales. That said, none of the material I produce is restricted to use in RPG Maker (or Visual Novel Maker). Some of my graphics packs are formatted to the specs of those engines, because they drive most of my sales, but the art can be used in other engines too. And in some of the packs I've included high res source files for even more flexibility. And again, with music, it's universal.