sure you are right no ponzi lasts forever
on the other hand some ponzis last very long

Instead of hourly payments i suggest daily. Lots of people will be happy with 1% return every day. But many people wont be happy to lock their coins in for 150 days. Its a tricky balance
But that will have the same issue towards the 2nd half of the payout period. No matter what the interval will be, the graph will follow the same falling curve.
The only way to make a ponzi last longer "fairly" is to manipulate these:
ROI % - Decrease ROI to lengthen time, Increase ROI to shorten time
Total time of Annuity - Lengthen/Shorten total time of payback of principal + ROI
Time in between payments - lengthen/shorten
Amount of each individual payment - increase/decrease
not sure if i'm missing anything else.