That rambles on way too long, but it does make a salient point: Space Marines are weird and bad, and attempts to make them just big manly fighty dudes that grunt and fight instead of weird mutant battle-nerds suck and are bad. They shouldn't be relatable, they should barely be human, and they definitely shouldn't be "wot if ur normal soldier man was big and extra fighty?"
Not least of which because Orks already fill the niche of being big silly guys that like to fight.