I think it's the product of popularity and the impact of the bug. Slack is exceedingly popular, so any change is going to come under a lot of scrutiny.

It is not clear to me how many new features Slack needs. Clearly people were able to successfully use the old text-entry UI; there was no shortage of Slack messages.

I feel like it would be better for them to focus on things that will add value to their platform without affecting people that are already using the core product. Meeting scheduling, corporate directory, video conferencing... nobody is in a better position to do that than them. And you can add those things, charge extra money for them, and not upset your existing customers.

Sometimes software is "done". The core of Slack is done.