Two comments:

First, I've been noticing a near-universal confusion about channels vs. teams, both among heavy Slack users and journalists. I think this underscores the complexity and novelty of the Slack-like mental model.

Second, how do you feel about using Slack for free? Let's make it a multiple choice question:

A. I know it costs Slack Technologies a ton of money to maintain our free team. I expect big, rich companies to subsidize our usage.

B. I'm well-aware of the costs associated with operating something like Slack. I'd be happy to pool $X/month for a community edition, but it doesn't exist.

C. It's not about Slack—it's about the new approach to communication it brought to light (IRC for mortals). If Slack dies, its niche will be filled out by a very similar free product—we will use that one until it dies, and so on.

D. Slack says it's free, so we use it. As advertised.