Whatever you think about cryptocurrencies in general, or the particular cryptocurrency chosen here, it seems completely clear that adding one to Signal by default is a really bad idea

Doesn't seem clear at all. Is it risky? I'm pretty sure it is. But if they don't and competitors do, it would limit the success of Signal and therefore the access/utility of on of the very few possibly trustworthy messengers.

And if MobileCoin is as good as people claim adoption of it would help establish a variant of crypto payment that isn't a disaster in almost every way.