You need some realism.
Any work of fiction that has no realism at all can't be understood.
That's why books about "Xylotirants who knoxilexes trgidians" don't sell, but books about "mystical knights who have to fight evil forces who want to suppress the whole galaxy work".
You need verisimilitude.
You need a frame of reference.
You need some realistic parts in order to support the fantasy parts.
That's why in like 99% of all fantasy and sci-fi books you have humans or at least creatures who are like earth humans. As a frame of reference.
And in the works that don't have humans at all (only kids movies come to mind right now), you have stand ins for humans.
And the D&D human is pretty much an earth human. That's why we are able to play that game, because if they were not human but Xylotirants, we couldn't even begin to fathom on how to roleplay them.
When we play D&D we need to assume that the mundane world outside of Magic, God's and other supernatural stuff acts like the real world, because if it would act totally differently, we would need all our brain capacity to imagine the mundane differences.
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I do believe this grounding is true. Although it doesn't have to have humans if other things -- emotions, gravity, hot/cold, etc. work similarly. Think Left Hand of Darkness.
But and this is a big but, that really doesn't have to extend to PCs.
Most of the world can be regular humans. But PC humans are also Wizards, etc. There is nothing stopping the magic world rationale talked about in some of these threads form existing and given these particular humans (dwarfs , etc) the permissions to have the ability to manifest fantastical abilities outside the baseline.
"The world is a magical place. Some sentient races manifest powers beyond normal members of their race and no one ones why. Often these abilities manifest later in life."
or
"The sentient races are all descended from the First People. Modern people are pale imitations of the first Peoples, who were more like Gods walking the planet. Some people have the blood of the First People flowing stronger in their veins and manifest powers beyond normal members of their race. Often these abilities manifest later in life."
Done. Normal humans (dwarfs, etc) exist and the ability to become a Mystic Martial or not also exists.
Now, I get the normal human does great things fantasy too, so I think there should be a peak human option that gets things like mandatory magic items at high level to allow for that archetype.
Also there should be an option at lower level to not manifest the supernatural stuff so you can be fairly mundane 1-7 or 1-10 then add the Mythic stuff later.