I agree that a lot of dino's may become pointless regarding their popularity but every single dino is unique in what they do (scorpion tranqs stuff, dilo's block vision, etc) so they generally all have a purpose, even if it's not exactly worth keeping as a long term dino.
About new players now being able to easily tame end game dino's (rex, quetz, mosa, etc) I think it'll still be quite challenging as the new kibble quality depends on both the size of the egg AND the rarity. Hopefully dino's like the rex require a bigger/rarer harder to get kibble quality. However, yeah, if they keep it so Rex still like scorpion-like kibble then they will be easily tamable early game because they will only need small kibble..