A few things to keep in mind, they could go out of business next month and yes you'd be screwed.
However most companies that seem to do this don't go publicly announcing it and advertising it. At least not the ones I've looked into, companies that provide these rely on their "subscription" customers. Offering obscure lifetime subscriptions can lead to a "lifetime" of advertisement if people like their service. Meaning in the long run more sales, theoretically.
I would take into consideration whether the company meets all your needs, see how long they have been in business, and check the lifetime pricing versus their regular pricing. For PureVPN assuming you bought 12 months at $4.16 per month it'd be 16 months before you got your money's worth according to their pricing.
PureVPN has been around since sometime in 2007, they actively block filesharing of copyright materials, they don't log, they have had their servers breached at least once a few years ago, they're based in hong kong. They don't appear to have a canary warrant, and there's probably a lot more information to look into but it might not be a bad deal if they meet your needs and you're willing to take the risk they could still be there in 16 months or 8 months if you want to use their $9.95 monthly pricing.