Meh, using the term "first born" for standard Astartes sounds like a sibling that can't find anything they do better than another and have to pathetically seize on birth order to find meaning and worth for their lives. "We get it Tomas, you made it big in the market, have a nice car, house, wife and kid, and all that, but I'm first born, so nyah!"
Parents in the background "Yeah, but we do still like Tomas more..."
Seriously though, they are all just Marines, and it's great that they have all been brought up to the same base Wound count - really hoping that we can see some more Chaos on the tables.
And people were saying that 9th Edition wasn't going to shake things up much... (oh wait, that was me, calling it 8.5). Glad to be wrong, really looking forward to seeing new army options/types come to the fore.
Or, you know, it could be a last bit of respect for the guys who paved the road for everyone who came after. The OG marines fought alongside the Emperor when the trators turned against them, and have stood against the tides of chaos and xenos threat for ten thousand years. They might be about to die out, but the new guys can pay homage to the fact that without them, Primaris wouldn't have even had a mold to break. Hell, these new guys don't even know Chaos Marines like the OG. It's almost sad that they'll have to continue a fight against twisted mirror images of a race of their former generation that doesn't even exist anymore.
Or, you know, we could do them like the Thunder Warriors and take them out back behind the shed, but I prefer the last bit of respect.
It's going to feel Really, Really good putting 2 wound Berserker's on the field in this edition. Daddy's home, and he has a lot to say to the newbies who have been having it way too damn easy thus far.