That's a fair point in general, but I think that by the time FB acquired whatsapp the writing was in the wall and I feel that the founders probably knew where things were headed.

This is the timeline as I recall it (and from quickly googling a couple of articles), feel free to correct me if I'm getting things wrong:

2012: this article

2014: FB acquires whatsapp

2016: Whatsapp becomes free (drops the $1 purchase fee)

2018: Jan Koun and Brian Acton leave FB