Well its definitely not good & perhaps not advisable to buy lifetime vpns. Its not a regular software that stays locally in our pc. Rather its real time with multiple online servers, somewhat like a secondary internet connection itself. All of it cost good money. Does any ISP sell lifetime internet connection at heavily discounted price?
Even if you consider from simple business point of view, if someone is selling a lifetime license product with free upgrades, that means to the person your value as a customer ends right after he sells you the product. Because there is no more financial gain to achieve from you in future. He can't sell you the same product again. Moreover, future support will no longer consider you as priority customer. So yes as you mention in OP, its like a Ponzi scheme. The only reason such lifetime licenses exist is because it is a good tool, without hassle & commitment, to secure funds for another better version of same product. A fire sale of lifetime accounts can generate enough money for the initial investment in developing another better product.
Maybe its why good vpn providers like Expressvpn or Nordvpn do not sell such license. You get to be like a "subscription slave" but you are treated well & get a good service.