I mean that when Red and Blue (or red and green) came out, when they were being designed, or even from all the early artworks and stuff, you can tell just by the eyes that they aren't designated as your friends, as these inherently human beings. They are feral, wild, vicious, or otherwise untamed from the moment you lay eyes on them. Even the wild caterpies and weedles and pikachus, they are simply animals with exotic abilities. When you catch it, you can see the potential for them to open up to you as you train them, and THAT'S what Oak is getting at with his post-champ spiel. But compare that to the designs since, where even if you were to completely neglect a mon, it's entire design still screams that they're tameable pets, not ferocious beasts.
Obviously, the intent is for any of them to be your friends, and I can't exactly put my feelings into the right words here, but what I'm trying to describe is the emotions, the feelings elicited when you see a pokemon for the first time. With the gen1 starters as my primary example, when you look at ANY of their forms, they all feel like even if you captured them, they might not ever become friendly, or tamed, or whatever. The designs instill the feeling that no matter how you raise them, there's the possibility that their cognition stays that of a base animal. But when you look at ANY of the forms of the gen7 starters, it's written on their face that they have human cognition or at least on an emotional level.
This extends largely to the trend of pokemon designs as a whole, but since this thread is about starters, I'll leave it there. I hope I was able to convey the feelings I'm trying to but I get that it might not be transmitted through text.