See, that's exclusionary. In order to make the most money, you need to have a broad spectrum advertising plan.
Not in the wonderful new digital world, it's all about narrowcasting and precise targetting now. The old mass market/Neilsens model is doomed. DOOMED!
If you target only those who already have Netflix, you gain nothing.
You gain added loyalty and less likelihood that they will cancel their subscriptions. The first job of a company like Netflix is to retain customers; then their job is to add customers. This is because the cost of adding a new customer is always greater than the cost of retaining one - it's the ounce of prevention/pound of cure idea.
I have no real idea what their marketing plan is, but it's possible that their target is only current Netflix subscribers (and Hollywood industry types). It probably does go beyond that, but they could have a very focused strategy based on affinities for premium serialized storytelling; dark political thrillers; Kevin Spacey fans; etc. I don't just mean targetting them within Netflix, but all over the internet. For instance, Kevin Spacey's Facebook page, or politico.com, etc.
Your idea about streaming old sports games sounds viable. I know new, live sports is a very pricey business but I'm sure there's an audience for baseball games from when current Netflix customers were kids, that they'd watch for nostalgia, or games from before you were born, etc. There's gotta be something like that already, and sure enough...http://www.freewebs.com/vintagesportsvid/
- doesn't look exactly legal, but it's the general idea.
The rights to that stuff should be pretty cheap, and it's the kind of niche audience stuff that Netflix should keep piling up. The gating factor is probably just staff time. Can you imagine all the specific tastes that they could be catering to? It must be endless.