It's the blurring of lines that some users are objecting to. Instagram is a perfect example where yes, important communication happens via Instagram messages, but the problem is that once the app is open, it's exceedingly easy to accidentally spend minutes/hours listlessly scrolling. In order to avoid listlessly scrolling, users will uninstall the app or set time limits on it, or other techniques to avoid it sucking up so much time. Compare this to a 'plain' messaging app without a broadcast stories feature - there isn't the same distraction factor and for those that abhor the algorithmic-feed and distraction factor, it's a huge turn-off to add that feature.

Go into Instagram/Twitter/TikTok/Snapchat/Facebook and try and turn off the stories/feed. You can't, that's the main engagement loop for the app!