Slack is going the platform route. They have a huge amount to lose disabling API access. Add-ons and interconnectivity are what make Slack so popular. Without all the integration Slack becomes much less attractive.
Other big social networks have a very different business model. Because they ask users to pay directly for the service, as opposed to ads on UI, using Slack via API or UI shouldn't really matter to them.
yes, you're right. However, the slack RTM API can still support IRC clients (with a little more work - need to setup the IRC server, like [1]). The API is like a superset that way.
