Note my previous caveat “But when your portrayal is supposedly straightforward realism”. Game of Thrones isn’t anybody’s idea of “straightforward realism”.
Everybody’s pretty well aware that audience suspension of disbelief in the case of fantasy or other “imaginative” genres, as well as in the sort of “innovative counterhistorical” variants I mentioned, is somewhat different from its counterpart in works that are intended to come across as mundanely realistic.
Nobody’s declaring that only “x” can play “x”. The point is that actors who really don’t look like “x” visually undercut the audience’s perception of “x”.
If you disagree, then would you be just fine with my example of platoons of short female actors playing tall burly male soldiers in purportedly realistic historical war movies? That wouldn’t come across as odd to you or take you out of the viewing experience at all? Yeah, right.
What I find disingenuous is this sort of strawman argument, typically trotted out only in defense of the casting of white actors as (same-gender) nonwhite characters, that theatrical art requires us to accept anybody playing anything because it’s “acting”.
You don’t even believe that yourself, as per your remark a couple of years ago to the effect that movie technology would allow the production of new Sean Connery Bond films with a (still living at the time) “de aged” Connery as Bond.
According to your supposed rationale, why would an elderly Sean Connery (assuming he were still alive) need to be technologically “de aged” to play a 40-year-old Bond? As long as he’s got the “acting ability” to portray the character, why does he have to look as young as the character?
Nah, you don’t really have a logically consistent position on this issue. And of course the resemblance criteria for lead actors portraying a known individual protagonist, such as Hopkins playing Nixon, are different from those for actors portraying characters whose only salient visual characteristics are general gender/size/race/ethnicity.