In fairness, this is completely zero sum so 100% is reserved for payments to 'investors' (better than even casino gambling which is known to have the best odds compared to most gambling such as national lotteries etc which hardly pay 50% sometimes) with no referrals or any other fees or commissions. It's true that if there are no new deposits, most people won't make any profit but they are guaranteed around 90% of their investment even if nobody else joins (contract balance and investment total is not much different as most of the ETH has come in the last few days - and it only pays 4% per day).

So even with around 1500ETH invested (slightly more than it has now) it only needs to pay 60ETH a day (4%). So I'm sure it won't just stop as it is still growing (it's now in the top 10 of all dapps on dappradar in terms of growth). Remember P3D went up and down for a while and then went up to 80K ETH. It is quite random, really, and depends on how many people hear about it. Less than 500 users so far which means it probably has only just started.

This is a good site with analysis of some (Easy Invest is the best at the moment):  https://beta.observablehq.com/@phantomydn/ethereum-pyramids-analysis

https://beta.observablehq.com/@phantomydn/analysis-of-eth-pyramid shows the analysis for this one in particular (not that I completely understand it)