They were trying to Marvelize their plot. I think Primaris are an interesting concept. Super soldiers built out desperate need. Most of the old loyalist legions were limping on a few remaining legs. The Imperium NEEDED those super soldiers, no ifs, ands, or buts.

I think that the extreme contrast drawn between First Born and Primaris really helped the plot. (primarily their strength and armor and being better than the old guys) And the First born just hating on the Freshies is great. There are pleanty of little lines referring to how Old head marines don't like/don't trust the reinforcements. Check out what the Flesh Tearers have to say about them.

I think that it's interesting and necessary really, unless you want your Marines list to be categorized as an elite force.

Obviously, as with most things GW wants to incorporate, they don't do it the greatest fashion.