The Primarchs are coming back, and that isn't necessarily good for 40k.
Don't get me wrong, I am happy that the story of 40k is advancing. I think it is cool that the Primarchs are returning. I am sure they will be amazingly fun in game, albeit likely OP, but that is not the point of this post actually. I am looking at this purely through the fluff lens. And I am saying that the return of the Primarchs, particularly the loyalists, is not good for the feel of the grimdark universe overall.
The Daemon Primarchs for the most part are okay to come back. Why? Well because ultimately they add to the grimness and darkness. They make the fight for humanity's survival exceedingly difficult. They are Chaos incarnate. 40k is ultimately about that struggle. Boil down everything into a single statement and it really is "Humanity trying to survive against the dangers of the galaxy."
But the Loyalist Primarchs have the exact OPPOSITE effect. They make 40k LESS grim-dark with their presence, and in multiple ways.
1- The are mythological. 30k is ancient history and borderline mythology (if not outright mythology) to the citizens of the Imperium in the year 40,000. While to us, on the other side of the 4th wall 30k is a set of cold, hard facts, it is not IN the setting. It would be like if Hercules or Achilles came back today, it would be kinda lame to see that they aren't all that we have built them up to be over the past few thousand years.
2- They represent HOPE, something there should be NONE OF in the year 40,000. Guilliman and his ilk represent a greater chance for the Imperium to not only hold off the oncoming darkness, but to turn it back. Definitely not "grim". I think this sums it up
To be a man in such times is to be one amongst untold billions. It is to live in the cruelest and most bloody regime imaginable. These are the tales of those times. Forget the power of technology and science, for so much has been forgotten, never to be re-learned. Forget the promise of progress and understanding, for in the grim dark future there is only war. There is no peace amongst the stars, only an eternity of carnage and slaughter, and the laughter of thirsting gods.
See what I mean? A Primarch returning flies in the face of that. Other than the Eternal War part I guess. But Guilliman is definitely about Progress and Understanding and not about living in the cruelest and most bloody regime imaginable. So he will likely try to change that (and not the Tzeentch form of change the Imperium is used to these days).
And even though this time only one has come back, he represents a big increase in hope that the others may come back. It isn't so much a legend any more, but a technical reality.
3- They lessen the achievements of the current heroes. Who are Dante, Draigo and Calgar compared to a Primarch. Lets not even bother with men like Gaunt, Yarrick and Cain. Lord Solar Macharius led a campaign that conquered 1000 worlds, but the Primarchs took the Galaxy. Sure, Draigo may have carved his name in Angron's heart and Dante clove a Blood Thirster in two, but next to a literal demi-god, even these supreme heroes are lacking. That's what was so great in 40k, you had men (albiet genetically enhanced ones) leading small bands and tiny armies compared to 30k MANAGING TO DO THE JOB GODS DID. Talk about the Baals on Dante! The contemporary heroes of the 41st Millennium are all the more amazing because they do not have the power or capabilities of a Primarch or Legion. This isn't even to touch on the fact that technically people like Dante and Lysander have been fighting for the Imperium LONGER than any primarch ever did, AND AGAINST CHAOS. AND TYRANIDS. AND NECRONS. Holy Terra! They are facing WORSE. Space Marine Legions never had to deal with Genestealers, never had to crash into the World Engine, never had to withstand 12 Black Crusades!
BUT Guilliman is still a bigger BA and will be portrayed that way from here forward. So thank you for your service current characters. Here is your participation trophy. There is the door.
4- AND VILLAINS. This has the same effect on our current set of baddies. Abaddon isn't a Primarch, but he has managed to do what no Daemon Primarch ever could. Kharn isn't a demi-god, but he has been Khorne's favoured champion for 10,000 years. Sure, we have seen Daemon Primarchs come back, but they INCREASE grim darkness by making it worse for the "good guys". But a Loyalist makes it worse for the villains, which has the opposite effect. Who is Typhus compared to Guilliman or the Lion? No one. And even other races have to worry about this. Guilliman probably can CRUSH the Tau Empire overnight. Necron Overlords are NO ONE compared to him. Ghazgkull is just Ullanor part 2. Probably on easy mode.
So with the rant over, I am glad they are advancing the story line. I just wish they didn't have to result to the Warhammer equivalent of "Gandalf look, its the eagles" to get out of this mess.
Though, those were some really sweet Eagles.