The Facebook/Twitter analogy is flawed, because those are ad-supported businesses, so the company has a strong financial interest in having users on its own/primary platform, where it can deliver ads. I think that's not the case with Slack (?). But even there, I think the incentive to not support N protocols is not to get the +0.1% revenue from IRC/XMPP users, it's velocity/simplicity in product development, which is worth more money in the long term.
Disclaimer: I worked for FB previously, on Workplace, which is a direct competitor to Slack.