Well, it makes sense from a fluff perspective that the Legions from 10,000 (30K) years ago would look different from modern (
40K) Space Marines. For the longest time, the Imperium has existed on the tech from the Horus Heresy. Nothing new has really been invented in those 10,000 years- just refinements and rediscoveries of old tech.
40K is supposed to be Grim Dark, with 10,000 year old technology- a kind of Dark Age. Just like the Dark Ages in our own timeline, science was eventually rediscovered and new tech made. It didn't happen overnight, but over a long time. 30K is the past, and should have a different aesthetic.
From a business stand point,
GW did invest a lot of money into making MK III and MK IV Space Marines, Cata and Tar Termys, but left all the specialized stuff to Forge World (where resin molds are cheap and easy to make). Maybe, eventually, like 10+ years from now when 12th edition rolls out, we'll see the last of those old Marines stop being made, with newer, larger true scale Marines being the norm. It's going to take a while though. When
GW converted Termys from metal to plastic,
GW said "You can play with your old metal, but here are the new and improved plastic!" You would be hard pressed to find anyone using an old metal Terminator compared to the plastic ones. Exception: That old Neck Beard that still talks about Rogue Trader and how great it was... For a while there, it just looked funny seeing metal terminators next to their larger plastic MK XII Space Marines and even larger plastic Terminators. I'm pretty sure that's what's going to eventually happen here. The new Marines will gradually move in, supplement, and then take over, Space Marines.
GW watches what sells very carefully, and when it doesn't sell,
GW is not afraid to cut the line, or introduce some new unit or rules to make you want to buy more of that poorly selling unit. I'm sure
GW is totally scratching it's head over Sisters of Battle.
For the Tomb Kings- That was probably a mistake. If you look at
AoS, all the old armies got "place holder rules", and as
AoS has moved forward, many of those old armies have been replaced with new armies- Sylenventh, Sigmarites, Fyreslayes, Kharadron, Ironjawz, Deamons... something was probably planned for all the Death Armies (Vampires, Tomb Kings), but with the way
AoS started out, and poor sales until the General's Hand Book came out, Death got lost or left behind. So all the Death stuff started to get phased out with an expected replacement that... never came. Don't worry- Tomb Kings are in the same boat as Empire, Dark Elves, High Elves, Wood Elves, Orcs, Goblins, etc. etc. All the "old" armies are losing models all the time. It kinda sucks that
40K 8th edition is coming out this year along with the Generals Handbook II for
AoS. I'm sure the rest of the year will be dominated by
40K, and we'll have to wait a long, long time for any more progress on
AoS.