In Slack's case I'm not sure how much this played a role. Slack itself really seemed like the kind of project a team of developers could write at a one week hackathon. So of course Slack was going to have to compete with essentially clones from all the major vendors and more, it's a very cloneable piece of software.

At least something like Zoom seems to not be as easily cloneable (the alternatives don't compare yet, in my opinion at least).