There's no such thing as an objective opinion. If it's objective, then it's not an opinion. But yeah, they were conservative with the designs. Probably because of hardware limitations, tbh.
There's a lot more to address here, but just quickly, objective opinion is an actual term. You're just taking the definition of subjectivity and applying it to disprove the existence of the other. But an opinion is just a statement that lacks and substantial proof.
For instance, I can't actually prove that gen 1 was conservative, but I estimate that the majority of people would consider gen 1 to be conservative or simple, even if overly so. I make this statement in objectivity. ie; I am not influenced by my own desires, wants, or outside factors. I honestly have no care for pokemon gen 1 or otherwise though I do appreciate its appeal and artistic style.
Objective opinion is an actual thing. While I'm almost certain I've heard someone on a forum or facebook say that the two words are contradictory, I would urge you to take another look at the definitions of all three and gently reassess whatever conclusion you were influenced into making.