If the protocol were to be upgraded then one would have to wait for all of the vendors to support the new features or we would end up working with the common denominator. Real world examples are hardly scarce, we see this thing in the open web with myriad (semi-) proprietary extensions. Samsung has also hard time getting developer support for their unique features.
In the end, the end users would suffer because of inconsistencies. Open platforms are good, but in my opinion they will always lag in user experience behind well executed proprietary solutions.
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?
If Slack falls then, well, we'll pay the price of migration, but seeing as it is a tool for ephemeral conversations I see it as less of a problem as migrating e.g. my e-mail.
For example if it was a rich client based on IRC, I could join in on a chat without being a slack customer. I could also be a customer of a competitor that would offer something similar, but more tailored to my usage. Now that would move the chat ecosystem further.
But that is not happening, because Slack's business model is hegemonic: they want everyone to use Slack to maximize profits.