Ok, as Leebo 先輩 hasn’t followed up with the explanation yet, I’ll try to explain, but I’m just a small clumsy cat, so my explanation would not be any good. Which is why, instead I’ll just post the extracts from the Dictionary and hope it would help.
Well, 野菜は、新しいほど美味しい。 must be ほど because this is the “the more X, the more Y” meaning (“the fresher vegetables are, the better they taste”), and you cannot express that using くらい. (This one’s in the Dictionary of Intermediate Japanese Grammar.)
I’m not sure why 彼女くらい親切な人はいない。won’t work with ほど.
Ok so regarding of the explanation for ほど, it’s more connected to emphasis things. That would explain the 野菜 sentence where the speaker refers to the taste and his opinion.
But there are 2 examples and the meaning is absolutely the same.
Today, I ran so much that I don’t want to run again in my whole life.
Today, I ran so much that I never want to run again.
One uses くらい the other ほど. I really don’t get the difference.
Also here? Why both? I would choose ほど because of the fact that the speaker is talking about the emotion.
彼女くらい親切な人はいない。Here, both is possible, but why?
Sometimes its interchangeable, sometimes not. And i don’t get what is the difference. I mean in all my examples instead of the 野菜 one, both are possible. pm215 explained this part but the other ones are still a mystery to me.