Don't think so and I've see a few guys say that.

We don't know much about semi and why washiro thinks he's the not the best choice, but i think the same wouldn't apply to oikawa. It has been emphasized many many times that oikawa is the setter type to fully draw out his spikers potential including ushijima. If you think about the way he kept on giving balls to mad dog, there would be no way where shirabu beats oikawa or in other words there'd be no way where shirabu has a playstyle more fitting to a team which relies on one super strong player. Even ushiwaka wanted to have him in his team and i don't think he's much of a smarty but volleyball-wise i'd trust him to have a proper view on things.

Sooo oikawa possesses the qualities fitting to be a setter for a super ace and by adding his serving (which is one of the best we've seen so far), his better receiving and strategizing qualities, i agree with OP and think that shiratorizawa would've been a favourite to win nationals.