While I completely agree that some people getting such high rewards everytime on their posts, no matter what they post, is wrong.
One day I was just going through the trending posts and trying to analyze what's in it that they are getting such heavy rewards and honestly, found some of them to be really absurd. I mean where I would not want to upvote at all. Again, that's a very subjective matter and I don't intend to disrespect either the author or the curators but we got to value the content that adds the most value.
That being said, I don't think capping is a really good idea because that would take away the charm of getting great upvotes. If I know the max I can get is this much, I would not put additional effort to get more. Some of us work really hard to get good upvotes and it's a great feeling to get a +100 POB upvote occasionally. And then the upvote value goes up to +150 sometimes. And it's so good to see that value rising. But if there's a cap on it, as an author, it would demotivate me to deliver good quality content.