An acquihire is not a success it's a failure, and instead of being excited, I'm sure the feeling is more like being resigned to at least having a place to go after closing the doors. It's sad too because their product is ace.
I do understand the situation better than most however because I've been there in a startup when the end is there and you're faced with acquisition to hire talent or closing the doors. You face it every day as time runs out and your runway shrinks. You face it a thousand times until you finally make a decision you were avoiding for a long time. It's not fun. So if these guys are happy it's probably because a weight has been lifted of their chest, not because they succeeded.
I would put money on Cover being a WhatsApp success story in 3yrs. Oh well, there must have been some internal issues we don't know about.
Hope someone steps up quickly to fill the gap!