A lot of assumptions that I think distract from the important - understanding the need vs solution first thinking.
The business might not be out to build their slack, chat could just be enough of a transformer alone. Besides, Slack is IRC client re-skinned with a bit different rendering. It's not the first either but maybe a business case is good enough to just chat as a big improvement.
Many organizations do just fine using something like Microsoft Lync, as much as I like Slack it's not for all users or instances.
The business might not be out to build their slack, chat could just be enough of a transformer alone. Besides, Slack is IRC client re-skinned with a bit different rendering. It's not the first either but maybe a business case is good enough to just chat as a big improvement.
Many organizations do just fine using something like Microsoft Lync, as much as I like Slack it's not for all users or instances.