I don't know, is that really true? Slack organises messages by channel and you can absolutely mute channels, prioritise them, control notifications etc. As for work/life, I saw people complain about that but you see people say the same about email. I think honestly it's sometimes the person wanting to complain. I have Slack on my phone. It's set to never send any notifications to me at all. I use it every six months or so, if I need to look something up or send a message on the move. Otherwise I'm on Slack when I'm at my desk and never at other times. No problem. Work/life balance has always been fine.
I've used Slack and Office 365 for the last five years and honestly find Slack better in almost every way, even to Outlook. Yes, Slack isn't the ideal chat app. Yes, it's stagnated and has no vision for where it wants to go. Yes once it finished cloning IRC and Skype they ran out of ideas. All true. But Office/Outlook 365/Teams is just so terrible. For years the Outlook Mac client struggled with the basics of formatted text! Groups are weirdly different to mailing lists, threading doesn't work properly, search? Forget it. People know how to create Slack channels but don't know how to create and manage Office groups so everything is just giant ad-hoc CC lists. Forget being able to classify messages by topic or importance because nothing is organised to begin with. Forget joining a conversation half way through and having access to earlier messages/files. It's an experience straight out of 1985.
But Microsoft is still crushing Slack: we're due to migrate off Slack to Teams in the new year. The sole driving reason is cost. We're not a big firm but Slack now costs the equivalent of a full time senior software engineer for us. Teams is "free" because business people consider Excel and PowerPoint non-negotiable. Microsoft is winning by market dumping in effect: it's IE vs Netscape all over again.
I think Slack could survive and thrive even competing against free, frankly. People do pay for products even when there are free alternatives. And so far everyone in the Teams migration trial seems to love Slack and hate Teams. Slack's problem is it has a massively bloated cost base. It's a freaking chat app, how on earth do they have 12 million customers paying such high prices and still lose money? It's abundantly obvious that they suffered the same investor-driven Dutch Disease that every other Valley startup seems to suffer from, where money is free so they feel obliged to take it and spend it. Slack's costs feel totally out of proportion to what it actually does, so of course they're going to get smacked by Microsoft. If Slack cost $10 per person per year instead of per month they'd be in a far better competitive position.