1) prospective contributors believe the hype and enter the market without doing any research
If I may comment on that, there are not actuall resources on failure and agency tactics. Just "how i earned {amount of money}" and referrals even to crappy agencies. And a total recycling of "news".
The actual research and evaluation perhaps is uploading and getting
rejected,
payed $1,37 per clip
etc etc.
You only really need to look at the total number of images on shutterstock and divide that by the total amount paid out to see the odds are against you. I'm sure though huge numbers do try it for a month or two and realise its not for them.
I'd have to dig through old saved files, but if I remember, 90% of the people with artist accounts on SS have no images and a much smaller group has 1,000 or more. Could be they join, upload, get rejected and discover it's not that easy. Or they upload and don't get a download in three months, so they pull everything and drop the idea.
I can't say I'm concerned with the all about total number of images as much as how many are the same as mine or better.
I don't care a bit if there are (these are real numbers) 1,423,808 sliced vegetables stock photos, because I don't think I have one. I do have 18 of the 374 XXXX stock photos, and a nice number on page one. Or another with 6 of mine on the first page, where there are only two pages.

And another I'm working on today: 758 XXXX stock photos where I have 6 of the first ten photos. Uploaded yesterday is now number 15 on the first page.
Make a note: new images get featured exposure on the first page!

If they don't sell, it's more about the buyers, competition and the images, than SS manipulating sales. Think about that folks?
Right, these are not "best selling" subjects, but if someone wants them, I'm on top and showing vs 27,730 spider and fly stock photos, which I have a couple and I don't have any idea what page. Hard to sell anything when buyers never get that deep into the pages.
You are right on the basic, most basic, outstanding point. The odds are against anyone new, ever making it like most of the people here did in the past. Uphill battle, millions of new images every week. Competition is fierce! Just hanging on, for people who have much better work and better images than I do, has become a hard fight. How can someone new compete with that?
New members can be featured but if they don't have better images, they are going nowhere. Anyone here can ask themselves, how long did it take to get the idea of what actually sells and what's a good stock photo for Microstock? And back to sheer numbers, if I have 5% of all the XXX photos and mine are on the first page and have been since 2010, I'm going to sell more! I don't care about 300 million images of who knows what else. They aren't my competition.