Are they worth 16B? None of the armchair investors here know their financials (addressable market size, user growth rate, expected ARPU etc). Until a careful analysis done, I would caution you against coming to a quick conclusion.
Slack for urgent matters, email for asynchronous communication that isn't quite as urgent. Email is also better for things that the recipient may want to refer back to in the future.
Regardless, I wish that my co-workers would stop sending a message only saying "hi, name" and then waiting for me to stop working to respond before they type their actual question. "not only am I going to interrupt you, but I'm going to make you acknowledge the interruption before making you wait while I SLOWLY type the actual incomplete question".
<Grumble, grumble, I hate when people misuse Slack>