I totally grasp the situation. The same could be said about notepad.exe, the File Explorer and everything else that comes preinstalled with Windows. According to the preinstalled logic, Windows should just be delivered with a kernel to even the competition.
AFAIK Slack is a Startup, backed by VC. If we would speak about Sublime Text vs. Notepad.exe, we might have ethical/moral grounds for a discussion.
Edit: … and just to be more clear: We have SublimeText, Directory Opus etc. - great software from great teams, which can survive although they compete with preinstalled software on Microsoft Windows. If Slack would provide something really valuable, Teams wouldn’t have had such an easy play. Same is true for Zoom.
IMO I don’t think “convenience” is a fair argument. If Windows did not allow their users to install another client, that would be one thing. But just pre-installing something on their own OS isn’t grounds for any complaint whatsoever. You don’t get to complain that Company XYZ makes a product A and bundles it with their product B and makes it more convenient for users lmao. Users have the means to install any client they want.