To a great degree, grindcore is death metal sans the deep death vocals and dueling guitar solos.
Respectfully, I disagree here.
Metal has always been a genre of melodic, long riffs put together toward a sense of the epic.
Hardcore, thrash, and their spawn grindcore have been about the literal reality, a kind of meta-politics that says: look at reality as it really is, and you'll see it sucks, and the disease runs deeper than our social institutions can address.
As a result, hardcore-derived genres are short riffs tied together by rhythm, with melody as an accent or tuning (Discharge) more than a method of composition.
Speed metal, death metal and black metal fused these genres and took the raw power of hardcore and applied it to metal ideals, sans the parts of metal that hardcore found disgusting (self-pleasuring, drunken, long solos, frivolous songs about fucking).
Grindcore has room to grow but if it grows too much, it loses that hardcore component, which I think is at this point obsolete -- it's too limited to be anything but rehashes of a few archetypes.
Ask yourself why there have been no epic hardcore albums since 1985, and no epic grindcore albums since 1992 or so.