Editing Primaris (section)

==Meta-History (aka "Why did GW do this?") ==
First of all, as a big fat disclaimer: none of us work at GW (and those who do aren't telling shit if they want to keep their jobs), so take this section with a kilogram of salt.

Now, onto the speculation:
# Across the years, there had been a massive amount of scale-creep. Pre-Primaris, a Space Marine figurine ended up not much bigger than a normal Guardsman, and with worse proportions to boot. So, they could really use an update to their looks to make them look more impressive and appealing.
# At the same time, there had been insane scope-creep: Marines had a unit (or a unit variant) for every single role you could possibly imagine. It made Marines Armies daunting for new players (and complicated for even veterans) who would have to learn and memorize all those variations of rules (if each model in a squad has different weapons, how do you even start with rolling hits and the damage?) And from a purely greedy GW standpoint, the unit bloat made it harder to come up with new models (not that the Priamris Marines helped this problem, as by 10th edition the Space Marine had more than doubled in size with even more specialised units).
# From the above point, Marines were ''also'' unappealing to new players from a painter's perspective: having to buy and paint so many different models, all with their tiny different weapons, heraldry and details, is terrifying to a newbie, and with their bad proportions, even a great paint-job would only translate to a "good" model.
# Because Marines are the posterboys, these are issues for GW, who think even more potential new players could be had if they do an entire range and rule refresh of the Marines to make them easier to paint and play, as well as more visually impressive.
# Because Marines are so widely played and used, GW can't just switch over their entire production pipeline in a few days. We're talking about removing/changing dozens of molds, hundreds of figurines, of an absolute logistical '''nightmare''' at the best of times, more probably an outright impossibility (and that's only on the "is it physically possible" side, imagine trying to tell to ''tens of thousands'' players that from one day to the next, their entire Marines armies are non-valid). There needs to be a transitional period where you can still buy and play old models alongside the new ones if you want, ''but also'', they need to make it clear those old models are going to disappear.
# So, they need to come with a lore reason as to why these new Marines look different from the old ones, why they have different weapons, armours and whatnots, why they fight differently, and basically justified the above point (and the transition period while they slowly expand the Primaris range and their production pipeline in the real world, which will take years).
#* Because it is mostly intended as a way to get new blood into the hobby, it would also be a good opportunity for a soft reboot of the franchise, an "other stuff happened before, but you can safely start paying attention at THIS edition and not miss anything too important".
# And thus, we get the Great Rift, the Awakening of Guilliman, the Unveiling of the Primaris Marines, and basically the entire lore development from 8th ed (which continued into 9th), all to justify why Marines look different now.
# Lastly this also helps Chaos. The Chaos Marines have, at times, struggled to have an identity beyond "marines with spikes". With Primaris Marines shifting to more fixed load outs, and Chaos keeping their wide-ranging, inconsistent, dare we say, "Chaotic" weapon selections, it finally makes the two factions have distinct game play styles. Marines are more "plug and play" with tabletop armies having dedicated units for given tasks, while Chaos keeps the all-rounder approach of older editions.

... Though ''some'' of those reasons are questionable, seeing how Chaos Space Marines [[Derp|just got scaled up models without having to go through the entire ordeal of making new armies with new codexes]]. Somehow, despite often [[Abaddon|suffering from even poorer writing]] than the loyalists, the Cha(d)os Marines can enlarge independently of their dad's magical return, Mechanicus Mary Sues, and unreasonable deux ex machina retcons. (Maybe if they'd been able to do it more gradually, like how the Orks [https://wh40k.lexicanum.com/wiki/Miniatures:_Orks got more swole] over the years?)

And of course, this all could have been solved by introducing Primaris as new Mk. X armor and assorted new equipment, explaining the tactics as Guilliman re-adopting the Great Crusade style of Marines being used en-masse, and thus with specialist units (while Sternguard act as the special forces/Veteran Tactical Squad equivalents still). They could have even explained it away with it not being that Guilliman can't change with the times, but that with the Great Rift and the 'end times' for the Imperium the time has come again for Astartes to be the forefront of the Emperor's field armies.

But then again, has anyone ever accused Geedubs of intelligent writing?