For one thing, the cost of licensing what they already have to stream is increasing, and with all of the consolidation that's happening in the industry, that probably won't be improving. Forget growth, they need cash to maintain the status quo. Also, the transition to all streaming is happening a lot slower than they wanted. They expected people to stop asking for disks in the mail, and it didn't happen (for obvious reasons). Mailing DVDs is expensive. Streaming them costs next to nothing. A big part of the reason for this change is to make people choose one plan or the other, preferable for them the streaming plan.
More important, we're talking about their current business model with a future that nobody can predict. Who the hell knows who people will be paying for their movies in 2 years time. Netflix has done a fantastic job of spreading their brand to a variety of STB's. Crawling into bed with M$ worked wonders for them. But honestly, the next big thing might completely wipe them out.