^I'd be extremely surprised if any of Danny Glover's performances were considered bad (hey, I'm in the minority that prefers Predator 2). But he's unlikely to be considered the definitive take. It's always going to be a one-off. Whereas Walter Mosely has been able to age Easy Rawlins, taking him from the 1940s to the 1960s and create his own private dick, who is probably better fleshed out at this stage than anyone created by Hammett, Spillane etc.
The Shaft movie is certainly dated but I think its significance can't be overlooked. Richard Roundtree was like a black Sean Connery, probably the first time a black actor could play a character like this onscreen, a swaggering sexy badass. The makers could arguably do more with him than they could have with a black Hammer or Bond. And he has a better theme tune than any of the 70s Bonds!
As for the other point, well, yes, I would rather see new characters created than 'bleaching' Asian or black superheroes. If they ever make The Falcon, Black Panther or Luke Cage movies, I want to see them played by black actors. I want to see Zorro played by a Latino actor, not turned into a red-headed Irishman (even if it gives me a shout at auditioning ...).