Slack certainly didn't win by having a good UI.
It has a good enterprise sales approach and integration features with enterprise systems.
It has a good enterprise sales approach and integration features with enterprise systems.
Also before Slack the competition was less Lync or such, but Mail, jabber and non-sanctioned chat (Whatsapp etc.)
The silliest part is you actually have the cause and effect exactly backwards in two different ways (nobody cares about integrations if they don’t want to use your product in the first place, and slack’s early momentum was specifically due to being freemium + product-led rather than enterprise sales)