Obviously I don't have access to "official" CGT data, but, in my personal experience, the Make-an-Offer feature was a major thing. According to my conversations list, I made 250 offers over the years, about 80+% were accepted by the seller right away, a few more were settled with a negotiated price (i.e. higher than my initial offer but lower than the current price listed on the website), and about 10-15% were either outright rejected by the seller, we failed to reach a compromise and I did not purchase, or I never received a response to my offer.
In other words, more than 200 of my offers led to sales. Not exactly all that trivial a number in my estimation. (I even had a few offers I made around Christmas accepted this years, after the feature had been retired.)
From my point of view the thing is this: Some sellers either don't participate in website-wide sales events (or only very rarely) and some go no higher than 30% off; but some of those in these two groups allowed offers to be made. Which significantly increased the chance of a sale being made to me. Now, after the removal of the offer feature, those sellers who don't participate in sales events will most likely never sell to me because I don't pay full price for items that list at more than 10 USD as a matter of principle (As a mere hobbyist I don' t NEED to buy anything; I'm quite willing to spend thousands of dollars a year on 3D assets but only if (a) the product interests me and (b) I can get a good deal; if sellers don't want to discount their products that is totally fine by me; no sale then. Thus, in my opinion, the sellers are more negatively affected by the removal of the Make-an-Offer feature than I am as a buyer; in fact, I'll be "saving" money by purchasing less than I might otherwise have.