The general consensus seems to be that we are in a race to that particular destination, so I would be interested in your thoughts/opinions as to where that destination might actually lie.
   I would be willing to bet that the agencies have hired consultants to answer this very question.  "How little can we pay them and still get enough new quality content to keep our customers happy?"  So I don't think we'd be tipping them off to any new information by discussing this.
   I'm probably the average US contributor with an average portfolio making a decent part-time income (can anyone actually live on microstock income anymore?)  My average RPD with 6 agencies is about 65 cents.  I think when it drops to around 25 cents, my incentive to produce and upload new content will be severely compromised.  But then again I'm a retired part-timer who doesn't depend on this to survive.  I think someone in Croatia who could live comfortably on my income and has fewer income options might have a different answer.

I don't believe in the race to the bottom because Pond5 and alamy are doing really well still paying 50%.  Istock is dead for a lot of non-exclusives.  Sold my highest priced licence ever with alamy last year.  That might of been a one off but generally prices have gone up since I started microstock and the sites that tried undercutting the market haven't succeeded.

Bottom for who?  If you mean when its no longer worth uploading, I reached that point last year.  If you mean when we stop making any money on our ports, its different for everyone.   I estimate 2-3 years and I'll be at or near $0.  I'm making less than a third of what I made in 2012.

For the agencies,  they can probably keep going like this for a decade or more for the reasons Paulie said.

The bottom for many are all over the place. If bottom for a contributor is moving onto something else or not creating anything new, my bottom from shutterstock happened last year. I've only been there slightly over 3 years and thats just to monetize off the files i made back in 08-09. i probably made less than 20 new vector files sold to microstock in the last 3 years. Most of my sales are from files i had in my istock exclusive days. and since i still make significant money on this, i keep coming back to this forum filled with bearish feelings.

When creating vector works for stock, I find a niche that sells. In creating my files, i spend a lot of time one each file. Probably 1 day worth of work on each submission. I produce commercial stock that gets many downloads and i'm happy because the time i spent is rewarded and the buyers get what they want to see. As the years go by, my talent has gotten better and when i put that talent to use and see no reward for it as my new files disappear never to be found by the buyer, i get discouraged and stop producing for that market. I saw this happen to me on shutterstock on more than 1 occasion last year and decided i no longer wanted to bother with that company. i don't want to be a sucker for a fixed system no matter how many talentless people think otherwise.

Gannett papers sell images uploaded by volunteer contributors with no compensation other than your photo is seen by many.  So, maybe the bottom is when they start charging us to sell our images.  Oh, wait, there are sites out there that actually do that already.  Never mind.

And that may actually be the true bottom. Where commissions are cut to the point where contributors only get bragging rights and are strung along being told they are providing a great service to humanity and saving the planet by sending in their images. Not just Gannett. Most newspapers do this to some extent. Does this sound familiar? "The weather was beautiful today. Send us your pictures of how you spent your beautiful summer day! (so we can put them on our commercial website and use them to attract more traffic and make more money without having to pay you, for a photographer, or stock photo)

The problem is there is a never-ending supply of new contributors who are happy with whatever is being offered today.

2010: New contributor "20% of $5? Great I'll take it" Existing macro contributor "this sucks"
2015: New contributor ".25 per subscription download? Great I'll take it" Existing micro contributor "this sucks"
2020: New contributor ".001 per subscription download? Great I'll take it" Existing ultramicro contributor "this sucks"

The bottom will be when the saleability of new contributors and their images decreases to a point where sites revenue growth stalls and they start losing customers faster than they can add new ones. But even then, a site can just adjust prices and also reduce contributor commissions to make up for the loss.

The problem is if this whole thing gets to the bottom and stays there.

The bottom is when an agency or one posing as an agency asks you to pay them per image or per month to sell your images on an experimental and speculative platform and asks you still  to promote yourself and them.

Silly or even shocking it is being done. Even more silly  or shocking is that people get involved with it.

Here's me two cents.

Large stock sites must have some fixed costs like storage of data, staff, legals, etc. So the bottom for some of these sites will eventually come. I can't see it being anytime soon. They still seem to be getting the lions share of each sale.

Rumour has it that one of them has large debts and others have shareholders which history tells us like to get paid, so there must be a minimum cost for them to keep going after which they should implode.