My rule: when buying a 'lifetime' service, assume that equals 10 years and weigh the cost accordingly. A lifetime Nebula subscription is $300, so $30/year. Is that reasonable to you for the service? Then go for it, and be pleasantly surprised if it lasts more then 10 years.
I do this too, but I use 3 years instead of 10. I think that's mostly due to the idea that lifetime memberships are most often offered by start ups that can be very short lived.
Is that legal? Surely they can terminate for convenience going forward if that was in the ToS, but they can't terminate a right accrued as of the terminate date?
It may not be legal, however rights holders will have to enforce their claims, which takes time and money. I'm betting they believe people will just take it and not sue them.