You get how groundbreaking Convenant was when it came out right? Grimdark is a very new thing in the context of fantasy as a widely written and read subgenre. And it's still mostly not as dark as actual myth and folklore.
So sure, before the 90's you have Convenant, and Moorcock in spots, and Gene Wolf's Black company, and a handful of others, but for many years they were voices in the wilderness, not reflections of the mainstream. That's why the comparison between high fantasy and grimdark is what it is, it's partially an old vs new comparison. Even the Sword of Truth isn't really Grimdark, it's more Fifty Shades of Tolkein. Anyway, this is all very sidebar and very subjective, as interesting as I find it as a topic of conversation. I'd happily bat it around ad nauseum, but it probably deserves it's own thread, and I'd bet it already has it's own thread somewhere here.