The reason slack just got acquired is that it was the best outcome for them. The reason that it was the best outcome is that they no longer saw a path to continuing to inflate their valuation. They were out of ideas and growth was slowing down. And since they took big piles of investor money, an acquisition was the obvious move to provide these investors the ROI they wanted. It's well executed; I think they managed to sell at their peak valuation.

Technically, what Slack did was provide a commodity with a nice UX that was easy to copy. The moat to defend against competitors was completely non technical: i.e. an enthusiastic user-base instead of patented hard to copy technology or a data lake. As such, that moat was doomed long term unless they would have succeeded in piggy-backing more products and features on the success. That never happened. MS, Google, and others waking up and building similar products was a matter of time and that happened some years ago. So, an acquisition was a foregone conclusion once the market filled up with many products doing more or less the same things.

Slack could have succeeded by doing something that completely went against their instinct of proprietary everything: open up and become an open standard with them as the reference implementation. Take Outlook as an example: it supports sending emails to people not using MS Exchange. That's not because MS likes doing that but because it's a core feature of email to be able to do that. They lost that battle a long time ago. Exchange actually started out as a walled garden and then email happened to become the thing that drove internet adoption. MS was left no choice but to add email support to Exchange.

Federation and open standards ensure that email survives more than four decades after its invention. Slack tried to replace email in the work place but they overlooked federated protocols as the key to long term success and survival. Of course success is subjective. I'm sure the 27 billion in shares is considered a great success. And who knows, maybe Salesforce will even do a good job of managing it.

But the reality is that chat is a commodity. Chat apps have come and gone for decades. Anyone remember ICQ? Did a great job in the late nineties. No longer exists. I actually used that at work. Yahoo messenger? All but gone. AOL messenger (later merged with ICQ), gone. MSN, gone. Jabber of course tried federation and for a while it looked like Google actually was on board (Google Voice was initially jabber based). Technically it's still around but I don't have a working implementation on my phone because I don't know anyone still using it. And I have just about everything else on my phone: slack, skype, telegram, whatsapp, signal, etc.

IMHO where Slack dropped the ball was with audio and video integration. They didn't do a great job and world + dog kept using alternatives even if they paid for Slack. So, instead Zoom just became the new darling of their target audience. They bought Keybase, which tried to be a secure Slack alternative. On paper the combination could be a Slack killer. And the data security features of Keybase could be a killer feature in e.g. the EU. Who knows, maybe somebody bright enough is in charge at Zoom to not that mess up. They have all the ingredients to learn from Slack and do better. But I'm not optimistic that will happen and predict an eventual acquisition is more likely.

So, many walled gardens and I still use email because it's the one thing everyone is on. You might call someone on Zoom; but you'll likely set that call up via email. Success would have been if you'd be able to use Slack to setup a Slack call because everyone you'd want to possibly talk to would be reachable via it. That never happened because it's a walled garden and they never stood a chance to replace email because of it. True for everything else in this long comment as well: ten years from now, most of that stuff will be forgotten and we'll still have email.