MS will screw it up
They will make a bloated clunky confusing mishmash that limps along while MS tells us it will work better if you only buy Azure and SharePoint. It won't work well with Android or iOS, it will have endless patches and unless your org is running a perfectly matched set of identical clients things will break. The beauty of Slack is that it precisely HASN'T changed much. It does what it needs to do, simply. They haven't tossed in 3 million horrible features no one wants.