Felix has put his finger on it. If it is indeed the case that s2Member is great for content protection but not for subscription management then it does not make any sense to compete against plugins that do it better.
My love of WordPress comes from its plugin structure which is the best implementation of the "small parts, loosely joined" philosophy that I know. s2Member is a quite a low-level tool and so I suspect that many users be using it in this way.
My own membership site is still in development and I have not yet worked out my membership use cases but thinking aloud they must include,
- Selling different subscriptions
- Managing upgrades and downgrades
- Managing end-of-life
Let us assume that we use s2Member for content protection. This content protection is actually implemented through WordPress Roles and Capabilities. A Subscription Management Plug-in would manage payments and change Roles and Capabilities accordingly.
Formidable Forms works well, it is a medium-level forms framework like Gravity forms. It has add-ons that provide basic PayPal and Stripe integrations. It would be necessary to build your own business logic for the membership use cases which could be implemented through custom form "Actions" (events that take place on Submit), these developer written Actions could make the changes to the Role and Capability of the user.