If Slack dies, Slack apps die so it is in everyone’s interest to provide a great UX on Slack.
There is why I see then tension in comparing Slack and Twitter’s API strategies.
Donald Richard

I don’t really agree with you here…

As the author mentions, slack invest in “bot-style apps” which is “loosely coupled” functionality to the core functionality. Is slack the only product or company that works with chat bots? I don’t think so, just looking around at Google/Facebook/Microsoft/etc — they all invest in bot-style apps too. Technically it’s also a re-usable functionality what developers make for slack — publish/subscribe model with push notifications, web hooks, etc.