XMMP was extensible to support all this in the early 2000s. Slack reinvented simple services in the most obtuse way. I have to use Slack and I sideline quarterback all the ways things could have been better every day.
Agree with this point of view. Except the Jabber/XMPP Cisco legal thing, there's just no tech answer on why on earth Slack did not use XMPP under the hood.
Slack would have known about WhatsApp architecture because it was widely talked about pre-FB acquisition (2014).
And Slack was founded in 2013.
Sometimes it's for OKRs and power/politics balance between departments and teams in an enterprise. "If an existing tech like XMPP is used then no serious development could be needed" fear (which is not really true). It can lead (and leads) to a huge waste of resources and overspending.
But it's a bit similar to building a luxury house. Not because an owner needs it. But because he can afford it.