Okay, I'll take the bait. I'm about as good a representative sample of the people you're talking about as anyone else.
I'm 39. I've been in technology in a professional capacity for 22 years, longer now than perhaps a few folks on HN have been alive. I wrote my first computer programs over 30 years ago, started online with local BBSs and then eWorld and then the internet.
I've seen a lot of stuff come and go. The tech industry as a whole has, over the last decade, adopted an especially frenzied pace that looks an awful lot like someone lost in the woods, freaking out, running in different directions trying to figure out where they've been and where they're supposed to be going.
There's nobody I can wag a finger at and say, "see, you didn't learn the last time 'round", because with a constant influx of new people, there's always a big group of folks that will adopt the latest thing because it's the latest thing, and have no memory of the last 30 years of latest things, and haven't been stung before when the latest thing fizzles out and takes a chunk of your time or infrastructure investment along with it.
Slack had advantages, sure. But it's hardly the first time that some service has come along and offered advantages over the tired old thing -- all except for a public, common protocol -- and then decided to take their ball and go home and to hell with everyone else. My staid desire for things being built on open protocols and available to people who want to self-host it doesn't come from some hardnosed ideology, it comes from years of, "oh no, this shit again."
Unfortunately, comments like yours tend to dismissively shout down the comments from the been-there-done-that folks, and so we're locked in a particularly self-defeating Eternal September that's costing the tech industry an incalculable amount of money.
In my more cynical moments, I wonder if a cadre of old farts oughtta start getting together and giving the less experienced folks the advice they want to hear, just to see how long it takes them to catch on: "building infrastructure out of walled gardens is a good idea, because they are commercial businesses and will be able to offer you more features than the slow-moving open source alternatives that haven't changed much in a long time".