If Microsoft buys Slack it would only devalue the brand of Slack, not improve the brand of Microsoft. Microsoft should continue to focus on the quality of it’s hardware products, to slowly become Apple.
Skype was the Slack of yesteryear. While it’s still used, no one thinks first to recommend others to use it. It’s utility. It’s only used when people have to, or there is no other option. Skype was purchased by Microsoft, it replaced Lync, it has chat options and Outlook integration, virtual conferencing, inherit (though non-permanent) channels, etc. Slack would be unnecessary.
Slack will be the same way. The attention and branding it has now is because it’s its own entity. Once it is enveloped, the users will no longer encourage others to participate… they’ll turn their heads to something else.
After all, there’s nothing really unique about Slack that wasn’t in an IRC client of the 90s.