I don't think OP meant
scheme, but simply a pyramid.
top = Sharks
middle = average players
bottom = money depositing fish
OP is asking, why do the money depositing fish who lose keep depositing, and if they stop will the pyramid crumble because the $$ base goes away.
Here's how I've kind of seen the
poker sites;
100K people, deposit $20 = $2M
10K of those play decent and triple their roll to $60 and decide to step up to a next level, but only $1K make it and triple their roll to $240, but the other 9K got a taste, so they redeposit (bringing in more money). And it keeps going until you have a select few at the very top who have slowly (sometimes quickly) made their rise and can stay there. Along the way, as they rise, they bring the bottom fish's money with them. Some go back down, then come back up, then go back down, then come back up. So, as long as you have new fish making $$, it WILL keep rising to the top. And, as long as you have fish (forever), there will always be new deposits made by the second, so there will NEVER be a lack of funds.
I myself am a fish. I have gotten a little better over time. Now, instead of losing my $$ within a week, I can make it last a few months... but (so far) it will usually go away. Why do I keep depositing? Because, what other hobby can I have that I can spend small $$ every couple of months on? To me, it's a form of entertainment. I would love for it to be more than that, but I've come to realize it probably won't ever be more than that, so I take it for what it is.
As the saying goes, "There's a sucker born every second", and 18 years later they will deposit their $$ on a poker site, every second. So no, the fish base will never go away and the pyramid will never crumble. Some will leave but, if it's a good site, for every 1 that leaves 2 will join.