History has it (at least in the 40k universe) that the old has a habit of being discarded and forgotten about. The Emperor first used the gene-enhanced Thunder Warriors during the Unity Wars on Terra before they were in turn culled for their service. The next project was the Adeptus Astartes, so it strikes me as very odd as to why being clearly inferior, they are hanging around. Perhaps those High Lords of Terra are now debating about their ethical choices in committing genocide against those Thunder Warriors.
From a background perspective as well, I'm really intrigued as to how many of the new Primaris marines have been created and how long it took. It seems really odd that during the Horus Heresy, Corax went through endless Indiana Jones-like temples just to have a chance at bolstering his legions numbers after the horrific massacre the Raven Guard suffered at Istvaan V and so many of the Emperor's secrets remained locked. Yet Roboute has found a way with the help of some Archmagos commissioned 10,000 years ago to come up with a superior version (I hope he wasn't paid by the hour).
The other question remains, just like the Adeptus Astartes were capable of falling to Chaos, how is this problem being addressed with the Primaris Space Marine? Chapters of these new brutes exist, which you would think if they fell to Chaos could then easily defeat a Chapter of 'lesser' Adeptus Astartes with brutal efficiency. But I'm sure the Lord of Ultramar has some 'practical' to counter this very threat.
Like GW has mentioned on their website however, I'm keen to see which Chapters have greeted these new brethren to their ranks with open arms and of course who has turned their back on them. The Blood Angels for example are considered the least trust worthy Chapter due to their flawed gene-seed, how does the introduction of this new marine effect this and of course with the events of the time-line in 40K can any of these new soldiers in fact reach Baal at all?
The Dark Angels are another Chapter that are well known for their dislike of outsiders, so how would they react? I'm sure the Imperial Fists would strap the latest in siege-breaking technology on the new big lads and send them to the front lines just like their codex following Ultramarine brothers.
I was interested to see a mention of a Primaris Redemptor Dreadnought as well. Wait a minute, aren't Dreadnoughts carrying near-deceased marines horrifically wounded in battle who can still serve the Emperor? But these newly commissioned Primaris marines literally still have their mark X tacticus armour paint work drying, yet there is a newly entombed warrior for every Chapter? Damn, who drew the short straw on that? It's amazing to see, that 40,000 odd years into the future, work place hazards still exist and some poor marines can't even walk out the door before copping a slug to the head.
Interesting times exist for the new edition of 40k, and just like every other long time fan, seasoned veteran or even on lookers, we're all waiting with baited breath to see what Games Workshop produces.