IANAL, so my take on this is amateurish at best, but when you buy a company you take over all their contracts, not just the ones you like. Whenever these lifetime accounts signed up, whatever terms of the account existed at the time still are valid.

So Id ask OP - do you have any of the original info from when you signed up? Any terms/conditions about the account? If you do, that would help clarify the situation.

Lifetime service has to either be so expensive that it’s paid for by basically an endowment (interest income on your original payment is enough to cover the cost in perpetuity) or simply a Ponzi scheme (your service is paid for by people who have signed up this year). There’s no third option (that’s sustainable).

The only other category would be situations where the service itself is now a money loser overall because growth stopped. Obviously exactly this happened to the original company and that’s how they ended up selling their assets.

For something as small potatoes as a cheapo VPN subscription, I’d be amused by anybody who wasted their time trying to sue somebody over this. I bet the company sold certain assets rather than sell itself outright. Meaning the beef of these “lifetime” users is going to be with a defunct, broke, company. Good luck forcing them to provide you with lifetime VPN service or a refund.