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.