GW looked at the newfound fanbase and success in Age of Sigmar and decided that they need to make 40k reach a broader audience. This is why there are numerous retcons and a drastic change in art style. Compare the image to the left (nu-40k) and the Karl Kopinski classic to the right.
One of the hallmark aspects of 40k, it's grittiness, is leaving the setting.
With the rise of Guilliman the Empire has been given a tiny bit of hope, whereas previously there was exactly none. All of a sudden there is a voice of reason and a mysterious new army of superhuman space warriors with all new technology suddenly rose.
This change is more dramatic to the lore than most people realise and it's all made to reach a broader audience and make kids' parents buy them shit for them.
Now I don't particularly care about the story and if it's progressing. To me 40k is a setting, but GW's new approach is trying to make into more of a story that is progressing and I'm sceptical what they're doing with it.