Only until Facebook decides to weaken or eliminate its end-to-end encryption. It's surprising that feature has lasted this long, since it's so antithetical to Facebook's business model of slurping up every bit of personal data it can.
It's likely there will be changes to WhatsApp soon.
Both WhatsApp founders have left Facebook recently, and they were much more privacy focused than Zuckerberg and Sandberg. My understanding is they protected the WhatsApp team from the culture of the rest of Facebook, but now that they're gone I don't see that continuing in the same way.
And just remember, those were the two guys who thought that transport encryption was too expensive for a messaging app.
I would like to be optimistic, but the realist in me thinks the number of users for which end-to-end encryption is a vital feature is vanishingly small.
And maybe all FB needs to do to keep those users is to make end-to-end encryption on WhatsApp opt-in, so they can provide ~~"wonderful new features to you"~~ if you keep it off.