All the changes HF21 brings are percentage changes
Free downvotes is brand new and not a percentage change. Some percentage changes can have a qualitatitive effect once thresholds are reached and it tips the system into a new balance point. We'll have to see to what extent that happens or not.
While you are correct to point out that quantitative changes can concatenate to create qualitative changes, free is simply less expensive. TBQH the downvote pool isn't free at all, since it comes at the expense of other benefits that would otherwise be paid from the pool, in exactly the same way that free healthcare, college, or any other welfare proposal isn't free.
However, I do believe it will result in qualitative changes. I foresee the development of a market for downvotes, and this will have an extremely negative impact on Steem society, as have all other automated voting mechanisms.
We will indeed see what comes of it.
That is a bad analogy. Free downvotes do not cost or expend any significant resources, unlike healthcare, college, etc.