This isn't as big of a deal as you might think. Once you purchase a subscription, you can cancel it anytime. Once you purchase, there will be a new option in the Tools menu called "Manage Subscription". The option to "Cancel Subscription" will be inside. Once you cancel your subscription, your card will no longer be charged. You are then free to purchase a new subscription with any new card.
In the future, I will likely add a way to keep the subscription and just change cards. For now, you just cancel and then subscribe again.
As a side note, you don't really need to worry about being charged on an expired card. Once the card is canceled or expired, Stripe will be unable to charge the card, and the TorrentRover subscription will be automatically canceled.
So fear not. You are free to cancel and sign up again.
As for an unlimited lifetime purchase... I don't think I can do that. For one, I don't know what would be fair to charge. I would have to know the future of the internet to know that I could support TorrentRover and keep it running far into the future. I don't know if I can make that promise.