Let's assume 25M subscribers at the standard DVD + Streaming plan, which I believe was $11/mo. We can chop off a few zeros and still get the same effect, so let's do that and keep our math simple.
25 * 11 = 275
I think the cheapest is now $16 for both, so let's figure out what they'd be making if everyone who stayed kept the same plan at the higher rate.
24 * 16 = 384
Let's derive a quick formula.
(24 - x) * 16 + x * 8, where x is the number of people who choose DVD /or/ streaming.
24 * 16 = 384 = (24 - 0) * 16 + 0 * 8
Now, let's solve for 275, to assume they'd be making the same amount.
275 = (24 - x) * 16 + x * 8
I put this into my handy equation solver because I'm too lazy to work it out in my head and can't find paper/pen...
x = 109/8, or 13.625.
Netflix could go down to 10.375M users of the DVD+streaming, and have 13.625M users of one or the other and still make the same amount of money.
Methinks Netflix did their math beforehand. They're going to be making bank, and savvy shareholders are buying now on this dip on bad news. Happens every time. Netflix is here to stay, for as long as the content owners will allow it to exist.