Think of it this way. Did the Web Browser need to be a proprietary closed protocol in order for the massive improvement over Gopher/WAIS or other end user hypertext information systems back then?

No, Netscape/Mozilla didn't need to "own" HTTP and HTML completely, and turn the Web into a giant App Store in order to innovate in the Browser client, neither did Google, Apple, or MS. At various points, proprietary extensions were tried, and either failed, or got standardized when they became popular or proven their need.

All Praise to Slack's UI, I love all the flourishes, like auto-detection of paste of code bits and offering to create snippets. I get it, I really do.

I just don't think we need another messaging platform owned by a single company.