As an extreme casual who only checks in on WH40k every so often, what exactly is up with the Primaris? I've tried to keep up with these threads and read around a bit, but it's just not making much sense. I've gathered they're not just new models, but considered entirely new units with all the requisite rules and cost benefits and armory and tech usage that comes from specific units. But they're also being pushed, both in crunch and in fluff, as being replacements for the Space Marines in general. But, why?
Why not just go "these are the new models, they're bigger now." Were the marines so fucked they needed to be completely rebuilt from the ground up and this whole endeavor is some kind of way to try and ease people into it? Why not just change the rules? Obviously they're peddling new plastic things to sell, but the way they're trying marry this with new rules and new lore just feels bizarre. Is it to try and get established players to replace their armies? None of what's happening feels like it makes much sense from any of these perspectives.
I just don't get it.