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What's interesting about the Preview Card discussion is how it evolved over time. The current status quo is pretty new. Once upon a time, previews came in random magazines, Magic's website (remember that damn orb), or, uh, were leaked and posted on MTGS.
When previews began hitting the web, it was for game stores that posted articles. That kept it to a pretty exclusive group in the preview circle. Here's Evan's take on that (I don't agree with everything Evan says here, but it's good perspective). https://twitter.com/misterorange/status/1339221900552237059?s=20
There's very little way to track the history here. No one was keeping tabs on it over time, half the websites that were involved are defunct (including WotC's own forums), and even today it would be a challenge just to figure out who previewed something from a set last year.
From my own recollection, around the same time Evan mentions the preview discussion above, there was a lot of clamoring on online outlets to let smaller creators have shot. The modern trend of TONS of small creators is extremely new, and the hope was that it would would help...
sites that weren't backed by stores have a shot at relevance. At the time the website had lots of articles every week, and those regular columns all did previews, and then there were special preview articles during the preview weeks.
Here's the problem, though, by the time smaller creators DID finally get a chance at preview cards, Reddit had created a culture that raced to post previews unsourced. In fairness, this was building before them from sites like MTGS, although not to the same extreme.
I can tell you from @TheVorthosCast's numbers, Preview Cards do not translate to additional listens. The first preview card can give a bump, but after that basically anyone who was going to listen is there already.
Amanda has an excellent thread on how Magic is pretty unique in the preview department in the way that small outlets get handed previews. https://twitter.com/AmandaTNStevens/status/1339200563674746882?s=20
