This is a valid point, but it's important to note that it only applies when using aggregate citation counts as a rough proxy for importance within a field. That's useful for prioritizing what to read but not for assessing the validity of any given result (of which there are often quite a few, at least in the life sciences).

Within a given paper, it's not at all uncommon for the authors to explicitly call out some detail from another study as being incorrect in their view. That doesn't mean that they necessarily agree or disagree with the rest of the cited work though.

What I'm getting at here is that negative citations would likely be far too coarse to be useful in practice. It's relatively rare that a paper outright disagrees with an entire work.