Eh, the term "grimdark" is largely misued. Actual grimdark is not gritty, but gritty "turned up to 11"—to the point of parody. Things like Judge Dredd and Warhammer 40,000 (where the term "grimdark" derives from) are the ur-examples of grimdark (where societal disfunction is exagerrated from something just beyond dystopian to outright farce and violence and/or war are not so much glorified as lampoonishly fetshized).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.
I think the rationale is based on two factors: some saves are "strong" (Con, Dex, Wis) and some "weak" (Str, Int, Cha) and each class gets one of each, typically in their primary stat and one other that tangentially related. So monk gets Dex (strong) and Str (weak) because Dex and Wis would be both Strong saves. The EK could theoretically swap his Str for Int save, but swapping Con for Int would leave him with two weak saves.Back to the topic though - class Saving Throws can be baffling sometimes. Why does a monk get Strength & Dexterity? Why does a fighter who plans to be an EK not have the option of Intelligence? I would entertain each class getting proficiency in one Save and choose a second. I would also entertain allowing the player to choose the Resilient Feat more than once, providing they choose a different stat.
Nope, I'm not thinking about adding that at all. If one of my players asked for a change based on a character concept, I would entertain it. They would have to work with me though. Like you said, a character can't have two 'strong' saves.So if you're adding a feature to pick your saves, I suggest looking at the last mystic playtest. They the class proficiency in Wis, and then the option to pick your weak save per long rest. Something like that might be option.