They're not asking for $4.99 -- from their perspective, once you sign up, you'll likely stay for years. $4.99 a month for ten years is $600. And you'll likely need to upgrade your iCloud storage at some point, because you'll run out of space, increasing your value even further. I wouldn't be surprised if many of the people paying for iCloud now will remain iCloud customers for the next 20 years, or even longer.