This is a capitulation - even Google uses Slack instead of Hangouts (although just a few teams); even Microsoft instead of Skype.
I wouldn't want to trust any business use to something that doesn't provide a documented API that I can use from my own code.
There are many businesses that still use Excel as their CRM software, even though there are purpose-built apps that work better.
My point is that Slack, Hipchat, and others are specifically targeting businesses. Having an API with integrations to other software (CRMs, issue trackers, PM, etc.) is one huge advantage over Skype, just as an example.