GW's sins are like scarlet, but I still don't get why the Primaris are so hated. To me, it just sounds like "old thing good, new thing bad."
Old 40k Imperium was Gothic Industrial. Shit was utilitarian, designed to work, designed to be mass produced, but also obviously slapdash and repurposed OLD stuff with some creepy pseudo-catholic vibes to it all. You had tactical squads wearing mixed MK4/5/6/7 armor because there was no way to mass manufacture mk7 (well, mk8 ) armor in such a way to outfit marines properly and they were doing whatever they could to throw infinity amounts of lost souls against the wall of suck to keep the (literal) demons at bay just one day longer. There were vibes of desperation, of cargo cultism, of a weird cross between religious overtones and scifi but tainted with shit like the best troop transport they have being a repurposed snowplow.
New 40k Imperium -- Primaris -- isn't Gothic Industrial. It's Tacticool Scifi. Everyone's armor looks the exact fucking same and they have ~3 variants and supposedly they can swap pieces between the three (Sergeant with a Gravis chestpiece but the rest Tacticus) but in reality that doesn't happen because the real reason is they only had to model and rig one blender file for each of the 3 variants and then it's just posing and adding optional cruft. They're not grimdark. There's no air of desperation, of repurposing shit that's as old as written language is on Earth, of really anything. Worse, we're supposed to believe that Cawl just happened to have hid an entire Imperium's worth of marines AND Mk10 armor and it didn't happen to come out until the shit was released nor did they bother to make mk10 sized for the non-Parimaris, or even give them the shiny new guns despite them being ostensibly better than the old guns -- not even as an optional upgrade, or something for HQs to use.
It's not 40k. It's something vaguely 40k-like. Temu brand imitation 40k. And that never, ever, bodes well for an IP long term.
The natural question becomes, "how could they have done it better?" Slower roll out. A series of novels where the big twist was marine power armor that won't fit marines cause it's about 10-20% too big and no one, not even the marines, knows what the fuck that's about but the AdMech showed up took it all away and are pretending they never were there and your ship's computer now swears up and down you never left drydock in the first place. Make it a mystery spanning an entire edition or two of 40k: Something weird is going on and no one's telling the Space Marines for some reason.
Strange sightings of pseudo-primaris marines in AdMech or High Lord Special Snowflake chapters like the Steel Confessors / Minotaurs or even the Death Watch and Grey Knights. Some gene-terrors keep showing up, maybe the implication something's fucked with the Primaris Geneseed -- imply or outright state that the AdMech relied on Hereteks or Xenos for help, or used some experiments Big E shut down for a damned good reason. Multiple instances of Proto-Primaris marines going batshit and needing to be put down. Tie it back to the HH with the Raven Guard and their geneseed fuckery, a book where Cawl is implied to have restarted that program or stole the information from it from Corvax without the approval or permission of Big E -- but Big E is indisposed and can't deal with him right now and besides maybe he can work with this, it sure beats the dumbasses rubbing WD40 on the keyboard cause they think the spacebar will be sad if they don't. Hell, give the Iron Hands something to do and imply Cawl was having them beta test the Primaris Geneseed in places with literal bionic killswithces installed in the marines' skulls. (Steel Confessors would probably be a better option for this but the IP rights of them are up in the air IIRC and they're Iron Hands successors anyway.)
AdMech obviously hiding something. Shit not quite working right. Space Marines slowly realizing they're about to get the Thunder Warrior treatment. Everything starting to boil over. But that would have been grimdark and can't have that in modern 40k, it scares the Alyssa Mercantes of the world away.
I would have also downplayed their size. If you want to redo the scale just redo the scale. It's not the end of the world. They should have paid to have someone do a complex blender file that had MK2-8 armor as options for each individual piece, in the new scale. That would have been far, far more useful for them long term. The worst thing is they didn't even get to do any line consolidation with the Parimaris like they no doubt wanted to do.