Yes, I believe the ability to stream episodes of The Simpsons will increase subscriptions to participating FXX cable carriers. The Simpsons has been around for a very long time, and has almost become a show with a cult following. People who love The Simpsons will switch carriers if the only difference between the two is whether or not you can stream The Simpsons.
The Simpsons is a classic and timeless TV show. Much of the people who are paying for TV subscription services today grew up watching the Simpsons. There will always be a certain nostalgia for characters like Moe, C.M. Burns, and Lenny. People will buy subscriptions to fill their homes with the happiness that The Simpsons provides.
I highly doubt it. The Simpsons is cool and all, but you can watch that online for free, so I don't see why people would want to pay to stream it. Cable is too expensive, a lot of people are moving on from cable to netflix, online streaming, and etc.
No, I don't believe the ability to stream episodes of "The Simpsons" will increase subscriptions to participating FXX cable carriers because of the readily available streaming sites that are already up and running. It may do a little bit for subscriptions, but I don't believe the increase will be anywhere near substantial.