But now we have Primaris Space Marines, which get ALL THE TOYS. From the gamplay standpoint, it feels like Grey Knights v2.0, and from lore standpoint, unless it backfires horribly and results in Second Horus Heresy (unlikely ATM), also feels like Grey Knights 2.0. MUH FUCKING BEST MARINES WITH BEST FUCKING TANKS BEST BEST BEST BEST BEST BEST.
Marines having all the toys ruins one of the more interesting things about them: that they aren't so much an army as a raging barbarian horde. Everything in their arsenal, from their ships to the Marines themselves, is built with the underlying assumption that Marines exist to punch the enemies of Man in the face; all other capabilities are secondary to getting Marines stuck in. Their ships love to fire Marines at ships and planets and so on; their biggest tanks are transports with guns. In the grim darkness of the far future, there is only rushing headlong at the enemy with fridge-sized slabs of angry meat and beating the crap out of them while yelling about the Emperor. The Guard handles everything else, and that dichotomy is interesting in a narrative sense. It makes it possible for them to be both awesome and fallible. It gives them reasons to need the aid of the tiny humans with shovels and artillery support. It also provided insight into the mind of the Emperor and humanized Horus slightly, in that the Marines are not only living weapons but living weapons specialized for a function that would be of limited utility in the empire they were created to conquer.
Primaris Marines, though, lose that fallibility, and not in a good way. They're just bigger better boring Marines that trade awesomeness for nuance, like they just stepped out of a propaganda poster. Now, you could argue that this is Guilliman's fault in-universe for replicating the mistakes of the father he so blindly idolizes (and Cawl's for not slapping him upside the head with a mechadendrite for being so myopic), but I also don't think a new Horus Heresy is coming so this is unlikely to ever be fully explored.
Then again, I mostly use 40k for running DH games, so I'm less concerned with the mechanics on the tabletop and more worried about narrative anyway. Maybe the Marines needed the Primaris to continue to be the flagship product.