The key difference is that life insurance companies, in general, tell their customers the truth about the nature of the product, while sellers of Ponzi schemes lie.
With a term life policy, the customer knows that if does not die within the term of the policy, he collects zero. If you cancel the policy, you stop paying premiums but you get nothing back. You know that the day after the policy expires or you cancel, it is worth nothing. The insurance company does not tell you that if you cancel the policy before you die that they will give you back your premiums and some profit on top of this, and then when the time comes it turns out that was a lie. They tell you up front that if you cancel the policy, you get nothing.
With a Ponzi scheme, the expectation is that you can cash out at any time and go home with a profit. You are not waiting for some essentially uncontrollable and unpredictable event, like your death. The seller of the Ponzi scheme tells you that you can quit at any time and walk away with a profit. But this is a lie. Some early investors make a profit, but most of the investors find when they try to collect that there is no money.
Term life may or may not be a good deal for you, but the time the insurance company honestly tells you what you are paying in and what you will collect and under what circumstances. (Well, honestly 90% of the time. As in any business, there are dishonest insurance salesmen.) With a Ponzi scheme the seller lies to you about how much you will collect and under what circumstances.
If a Ponzi scheme operator told you honestly up front, "I'm making payouts to early investors using money collected from later investors. If you try to cash out early, you can make a nice profit. If you wait too long, there won't be any money left and you'll lose your investment", and you decided to invest on that basis, I suppose it wouldn't be a con game. It would just be a very bad investment.
As @DStanley says, the fact that some insurance companies miscalculated expenses doesn't turn an honest product into a con game. The way insurance works in America, it's unlikely that any customers will not be paid the benefits they were promised. If a department store calculates that they can make a profit selling me a toaster for $15, and so I give them $15 and they give me a toaster and I take it home, and then the store discovers that it really cost them $16 to sell me this toaster so they lost money ... it's difficult to see how you could say that I was cheated or conned in any way on this transaction. Too bad for the store that they lost money, but that's not my problem. Presumably they'll raise the price so future customers will pay more, but again, not my problem.
I suppose that if a Ponzi scheme had enough new customers coming in, and a large percentage of the existing customers didn't try to cash out, the scheme could, theoretically, continue forever. In practice, for a Ponzi scheme to work you have to have ever-increasing numbers of new customers. Eventually you run out of people in the world who could join. But in any case, a Ponzi scheme is based on a lie. If someone gets away with a lie and lives out his entire life and goes to his grave without his lie ever being discovered, he's still a liar.