Is it easier to increase your rankings by a) building solid quality backlinks to your site or b) building tons of spam links to your competitors' sites?
If the answer is b), which I suspect it might due to reading some comments here citing fiverr examples, then allowing negative SEO is decreasing the quality of search results.
Also, my guess is where the damage is really done is w/ long tail searches. That's when I almost always encounter random, irrelevant, spammy sites - when I search for something very specific in Google.
This reminds me of a product features framework I saw once. It mapped features to a 2x2, which categorized features into 4 buckets. The lead bullets were considered "table stakes" (core features that were necessary and everyone had) and silver bullets are probably "fool's gold" (features that are distinctive, but don't necessarily drive adoption).