While everyone has their price, my point is a little more subtle: they probably could have had their price met and also codified a set of post-transaction terms

While everyone has their price, my point is a little more subtle: they probably could have had their price met and also codified a set of post-transaction terms that preserved their vision. For a founder-controlled company that raised capital so late, is selling for $19 billion much better than selling for $15 or $13 billion and knowing your creation will solve the problem it was meant to? Any of these outcomes leave the founders with many billions (and leave the few employees set for generations). Contrast with a $2-3 billion acquisition like Oculus or Instagram. They were huge but still in the range where 20% or 30% less (for restrictive deal terms) might actually matter, if only barely. Whatsapp's deal was so large that there's not even the tiniest practical tradeoff from 20% less.