WeChat and Indian "Super Apps" followed this model to grow to success. WeChat Pay in 2013 was basically like Facebook taking WhatsApp and using it to become a player competing against MasterCard/Visa.
Yes you can build a giant do it all monolith app to duplicate your competitions features and then crush them. The fact the apps are packed as a single program instead of via distinct programs isn't THAT meaningful and I'm kind of happy it isn't otherwise we would veer into lawyers designing programs more and more.
The Slack/Microsoft thing smacks of anticompetitive behavior simply because Teams is essentially free and on Microsoft's lowest cost plans and Microsoft is transparently attempting to crush Slack to death in a price war to secure a monopoly. The underlying quality of the two products is rendered a moot point.