It's not that downvoting doesn't have downsides, they are considerable. It's just that without a modest amount of free downvotes, we don't really have a realistic chance of turning this place around at all.
Currently, we're paying content indifferent voting behavior (self vote, vote selling) 4x more than curation. When we bump curation to 50%, there's still a 2x gap. The modest amount of free downvotes are further designed to bridge that gap.
I'm one of the ones who recommended these specific numbers for the EIP and I can tell you I'm very aware for the adverse effects. Let's say that at any given time, under the EIP they'll be around 5,000,000 SP worth of whales consistently being abusive with their downvotes on purpose. 25% of that is 1.25m SP out there making everyone's lives miserable.
Now look at the flip side, instead of next to nothing, if everything works out, you could have 100m SP worth of upvotes being cast in a relatively honest way that is reflecting their appraisal of the content. And half that money will be finding its way into the pockets of good content creators.
Maybe my numbers are a little optimistic, especially the latter, but overall it seems like a good trade off. We can't focus too much on the negatives alone without looking at the positives.