[ikonst]

WebRTC is somewhat new and complex, and browser support varies greatly. I don't know what kind of unlimited resources behemoth you imagine Slack to be, but I can definitely see how doing consistently working cross-browser WebRTC would be too much effort to be able to pull off at a given time.

[pcwalton]

This is apparently largely because Chrome's implementation of WebRTC has been less standards compliant than that of Firefox. See https://twitter.com/adambroach/status/959298097674305537 (by one of the WebRTC developers).

[zoul]

$250M sounds pretty behemotish?

[ikonst]

Maybe not for the operation they're running, of which VoIP is only a small part.