If you think that anyone could have walked into those roles in the original series, I would refer you to the many fan productions of the original series.
Most of those guys aren't professional actors, and no one said "anyone could have". He said the new ones are better actors "for the most part".
I'd think that this line you passed over heavily implies that that is not the case:
LEILA: Mister Spock's feelings were never expressed to me. It is said he has none to give.
I think you are the one who missed a line, namely that this does not preclude private feelings or other shows of caring. STID makes that very clear.