Even without Slack's ransomware-as-a-service business model, being dependent on a specific company is a big problem. If Dropbox decides to ban your country for being running too many AI crawlers, or DejaNews goes out of business, or XOJane decides to pivot to a different market (see http://web.archive.org/web/20171015000000*/http://www.xojane... for example), or LiveJournal gets bought by Russian trolls, it's potentially a big problem.