In the last 5 years, Whatsapp has basically replaced SMS in many EU countries. The equivalent in the US is probably the combined market share of FB Messenger, Whatsapp, iMessage and Signal.
And if it went away it would be inconvenient for a couple of weeks as people move to Matrix :)
Yeah, I don't think Facebook wants to let people see how quickly they'd adapt and overcome to any of their services going away.
There was a forerunner of that last year when there was a massive exodus to Signal when FB changed WhatsApp's privacy policy to force users to allow sharing of data with Facebook. If they shut down, even people who were too apathetic to switch would do so overnight. I hope Signal has a contingency plan for scaling up 10x ready...
Is the sheer amount of centralised scale (and convenience for users) of whatsapp a good fit for a decentralised system like Matrix?
Whatsapp is often (not counting groups) easy to replace. Your part of the social graph is literally in your phone's contacts.