The tie-ins with work flow management tools I get (even though most of this can be set up in Discord as well) but I think the look and tenure of the application is not really that big of a deal. If you want professional looking from a company that has been around forever then pick Microsoft Teams, otherwise pick the best one. And I have a hard time seeing Slack as better than Discord.
Just because you don't care how something looks doesn't mean that's true for everyone.
Fashion and style matter to lots of people. If 2 things are roughly equivalent, the one that's cool and looks neat is going to be the one chosen by the people who do care about those things.