The Primarchs are coming back, and that isn't necessarily good for 40k.
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.
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.
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.