Slack is half-way decent, so big corps queue up to kill it off
In my day jobs, I can use a total of: Slack, Lifesize (which has chat), Skype for Business, Microsoft Teams and quite probably a couple of others (one place I worked at used IRC).
Out of all of those, Slack is pretty good, IRC is plain and simple but effective and the others are all awful. I wonder if the purveyors of the others though "we've got to be different, so lets take all the good stuff away"? They're so terrible as to be unusable - pick up the phone or send an email instead.
Since Slack has a large user-base, it's obviously ripe for a takeover. All those lovely users to assimilate into one's own crappy platform - too much temptation to bare, I expect. The thing is, if it goes anywhere near Microsoft, then there's no way it'll be allowed to carry on as it is, it'll have to be subsumed into an inferior product. If Amazon get it, then there's a chance it might stay quite good, at least for a while.
Either way, Slack, we love you, and we'll be sorry to see you go :-(