One of the reasons why legacy apps continue to live on (despite outliving their usefulness ) is third party tie-ins.
The reason most companies are reluctant to switch from slack is because they have external third parties i.e. their partners connected with external channels within slack. Changing their internal DM/IM system will remove their ability to communicate with their partners in the same system.
Perhaps a plugin where slack messages can be forwarded to Chatto and eventually bi-directional i.e. Chatto messages can be forwarded to Slack.