Additional features that can't gracefully degrade would be a selling point for slack. "Your client does not seem to support video calls, we do, purchase an account here".
When it comes to file transfers, a simple link to download the file from slack.com is a good degradation example.
Once Slack is installed and the norm, you lose some choice. You're prisoner of the way they want chat to be.
I'm sure glad Firefox exists, because otherwise I'd have to hop in the Chrome bandgwagon and sell my data and habits to Google without a choice. The competition there goes further of course, as Mozilla upholds different values, and competing teams try to make their products faster and better.
And then, they fall. Look how Microsoft used to sell a product but now capitalizes on users' data. What will happen when Slack falls? What if I don't want my chat data hosted by some company, but in house, and my business partners use Slack? Where will the choices go?