It would be much simpler to just have a proper billing mechanism to begin with. Using additional software to manage accounts or managing sub-sites is not really an efficient use of my time. You would still be manually managing access to those accounts. Even if you only have 5,000 subscribers, that can be a headache. I would much prefer to pay someone to develop a proper non-paypal gateway than to get involved with devoting hours of my time to enabling or disabling subscriptions and managing multiple installations on the same site.