Most of what I'm about to put in writing has already been stated plenty of times over. Today, I wanted to chime in with an old timer's point of view. As my long term readers will be (painfully or otherwise) aware, I have been involved in this hobby since Rogue Trader days to a greater or lesser extent.
Over that time period, I've seen a vast number of things change. Not just rules, but the setting as well. Back in the old days, Tau were nowhere. Necrons did not exist. Tyranids were gaunt looking things. Genestealers were beasts that lurked on space hulks and infected whole planets much as they do right now, but were not connected to the Tyranids. Unlike Zoats. They were a Tyranid slave race. Eldar were pirates. They didn't have any aspect warriors at all. Orks were orks. And some of them liked Khorne so much that they worshipped him. (some were also hyribized by genestealers). As did some of the Night Lords. The Alpha Legion might have been Slaanesh fanatics. The Iron Warriors might have been slightly too.
Many people will contend that the 40k timeline has never evolved. I would agree, but with the critical caveat that it has evolved by ret-con to a strong degree. Eldar were once piratical maniacs to be feared. But now they're the seed of a new victory over the chaos gods. The necrons were merely asleep. The Tyranids were always on their way, you see.
With 8th edition, the timeline has truly advanced for the first time in a long time. The return of Guilliman is simply unprecedented in all of 40k. And with him, the ten thousand year long plan that no one ever heard of, or discovered, to build better space marines. The Primaris Space marines. But they don't fit in rhino tanks much like terminators don't. Drop pod armies also seem to be a thing that is about to be dropped in to the past.
The new 40k seems ripe with new possibilities. Although I am cautious about the story line being advanced significantly, I actually don't mind. I think this has been coming for a while and I approve of it. I will be looking forward to seeing where it actually winds up in the months and years ahead. Will they truly consider other sweeping moves like this? Or is it better that we all burned in the fires of Horus' ambition after all?