That's because the thing that killed Slack wasn't Microsoft - it wasn't a bunch of horseshit executives say just before their retirement into angel investing - but Discord.
Discord has like a $2billion valuation and no exit. Slack had a $24 billion IPO. Consumer software is not as valuable as enterprise.
As a business with employees sure, but as a community? You couldn't have moved everyone to discord faster
The only ones who I haven't heard lament the switch are the ones whose budget Slack used to (and o365 does) cone from.