A small-but-meaningful number of 'paying customers' aren't themselves users. They are paying the subscription price for the value of community peace, not any specific content.
For example: I pay for a Netflix account. I never use it anymore (since Netflix keeps killing the only shows they made that I liked), but my kids use it sporatically-but-heavily. (i.e., maybe one or two hours per day 4-ish days a week, but all at once). If Netflix makes it break for my kids (like they're threatening to) then I might as well cancel it anyway, it's worthless for those 'kid tv breaks' now.
It sounds like a small-but-not-trivial percent of Netflix subscriptions are something like mine (paying to keep other people happy, paying to prevent people from bothering them, not to watch any specific show/movie). And Netflix is playing in the "fuck around" part of "fuck around and find out".
Combine that with europe getting less content than the US, with bad or no subtitles, and the quality of the content going down the last few years. People are discussing cancelling already. Forcing these people to decide now can't end well for Netflix. And if people around you cancel without much hassle, why wouldn't you?
There will surely be some, but the default case is likely "grandma stops watching Netflix". That's an "exodus" of people who weren't paying them anyway (and were consuming bandwidth). In a few cases, they'll pay more money for grandma. Netflix bets that this category is bigger than the one who quit.
Why would paying customers quit? To teach Netflix a lesson? I suppose it happens. Because they think AppleTV/HBO/Hulu is a better deal? I suppose that happens, too, but we're talking about products in the range of $10/month (or ad-supported) -- not exactly the kind of thing where people are going to find massive bargains.
There will be some exodus, to be sure. But I don't see any reason for a mass exodus.