Am I taking crazy pills? Are we really wondering how Netflix is going to survive because EVERYONE has subscribed? Isn't that the problem everyone wishes they had?
Also, sub/unsub is not new. Everyone did the same with HBO and they did just fine.
Not necessarily everyone, but saturating their addressable market.
Imagine this simplistic model of Netflix's finances: they pay ABC $1M/year for ShowA. They have 100k customers in year 1 @ $10/mo, so (ignoring operational overhead) they can afford the show.
Year 2, Netflix wants to continue paying for ShowA because they want to grow their catalogue. Let's say the price has dropped to $800k because many shows lose value over time. Netflix adds ShowB for $1M, meaning they are paying ABC $1.8M. They now have to add 80k customers to be able to afford the new show + the old show.
Point being: This is highly simplified, but the rate at which they can add content is bounded, in one way, by the rate at which they add customers. In other words, if they aren't growing their subscriber base, the existing subscribers are just paying for a catalogue of content they've already watched, or Netflix has to start expiring old content in favor of using that revenue on new content. The second path is obviously the better one.
This is a fundamental thing in services like this, which is why no single service will ever be a "complete repository" of every piece of content in the world. Individual services can only be complete repositories of content that the company which manages it owns. Which is exactly why I'm bullish on Disney's new service; Disney has the most powerful content library out there, between all of their properties. This service will do extremely well, likely better than Netflix.
I think you are vastly overrating how much Netflix must pay to have old shows. To many content owners the choice is between not monetizing their IP, or licensing it to Netflix. That's a pretty weak position from which to negotiate price. Also, content is usually licensed in bundles, which is win-win for content owners and Netflix: The content owners get to monetize their low value content (which Netflix would have otherwise declined to pay for), and Netflix gets a lot of content by paying only a little bit more.
Also, sub/unsub is not new. Everyone did the same with HBO and they did just fine.