I think the main points of criticism about Slack are not the ones you listed. It's that it's a proprietary protocol, a data silo that can't easily be accessed,

I think the main points of criticism about Slack are not the ones you listed. It's that it's a proprietary protocol, a data silo that can't easily be accessed, the bait and switch of first supporting IRC and then killing off the gateway, forced to use the official electron/browser based client. I never heart anyone complain about inline images / gifs (which is also something that most IRC clients support) or that it's paid (which IRC bouncers / clients are often too in cases).