I'll point to oldhammer examples, the classic Battlefleet Gothic Cover from Jon Blanche (Army Painter Won!) and the Cadian painting by Karl Kopinski I posted previously:
Grim. Dark. Blanche's art needs little elaboration but the Cadians really do a lot of work. The composition over a white field is evocative of how historical uniform reference manuals will to illustrations of soldiers, and their poses are evocative of war photography, WW2 especially. The state of their uniforms also gives the sense of grizzled veterans, as does the random shit attached to their battle rattle. I especially like the shock trooper's ankle knife and holstered stub gun with extra rounds. Also worth noting that while the models never had this much kit, they're carrying an appropriate amount of bags, grenades, pouches, rolls and other crap, making their status as hard bit shocktroopers obvious. This image works really well for the guard because while contextually this is from a force description for Codex: Eye of Terror featuring a man wielding an industrial strength laser welder and another with a saber lightsaber as weapons the framing and visual language reads them as regular soldiers to us, despite the differences.
Now lets do a stylistic comparison to a modern primaris illustration with some tasteful accompaniment from D&Dt:
I would argue what you are seeing as "professionalization" is subsuming an idiosyncratic style and tone to tabletop industry standards to presumably mass market appeal. Note the cleaner more hard edged look. The earlier guardsman are not easily visually readable, and you might've had trouble perceiving one was a Kasrkin without taking time. Further I highly doubt Jon Blanche could tell you close to what a 1/5th of the bits on the space ship he painted do, because he's not a technical illustrator. The primaris marine images to me just read "this is a marine with primaris equipment and the obligatory purity seals and skull so you didn't forget this is Warhammer." Kopinski's Cadian's communicate grimdark through visual language and context despite having less skulls and Chaos per man than most primaris marine illustrations I've seen. Yeah, Warhammer fits in better with D&D, Pathfinder, and other digital media atrocities, but at the expense of style and the opportunity to communicate tone and themes.
You can see the same thing with fantasy illustrations, like those done for games like D&D over time. I would spend more time picking example images, but I've spent too much time belaboring my thoughts on warhammer's artstyle enough.
