We do know that phones and tablets are vulnerable - so it's not like we're unaware of any backdoor that may also be used to subvert whatsapp.

It'd indeed be interesting to know if the FSB had some kind of baseband vulnerability that they'd used willy-nilly to facilitate dragnet surveillance.

I suspect William Binney was right though - blanket surveillance is just expensive and hides your needles in a mountain of hay; you really want high quality in the data you store in order to ease extraction of meaningful information / intelligence.

(that's not to say that aggregate meta data isn't interesting - just that with actual content noise is a problem)