I'm not a big fan of the argument "movies have been terribly cast in the past, so there's no reason they shouldn't be terribly cast in the future."

Shohreh Aghdashloo should play Navani because she looks like Navani, (i.e. she looks like she's in her 50s because of plastic surgery and magic-youth-money), but a "normal" 68 year old should not play Navani. Navani is 50. The fact that movies have made a terrible decision in the past doesn't mean that we should encourage making terrible decisions in the future.

(By the way, by the time any Stormlight Archive adaptation even exists, Aghdashloo will be well into her 70s at least, so it will be even less relevant)