This is one of those problem statements that just doesn't resonate for me at all. I really like things separated into the different buckets so I have a good idea how to find it again later.
The idea of combining stuff into one feed is kind of a nightmare for me.
In Struct, you can create custom feeds, which can filter threads by channels / users/ tags, etc. You have that choice. In Slack, every one of these entities (and permutations for DMs) stands alone. It's set in stone, unchangeable.
That itself is a big win.
In fact, feeds are everywhere. That's how we consume almost all internet. RSS, News, Facebook, Twitter/X, Instagram. Feeds scale, they are powerful, they work. Instead, Slack has us jumping around channels and DMs for drips of messages, playing Whac-A-Mole.
I like the channel based structure of Slack. It gives me a confidence that I can always go back and find any message, it won't disappear, or be unfindable.
I don't like the "filtered feed" model of Facebook and Twitter, because I'm never sure what has been filtered out. And the fact that everyone is getting a different view makes it hard to develop or maintain shared understanding with others.
Feeds are fine when it doesn't matter if you miss stuff - jump into Hacker News, read some of the latest posts, then disappear for a bit. Not so good (for me) when missing things matters.
There might be interest in building a smarter feed which uses recommendation algorithms (and therefore have different views for different people, assuming same channel membership), but that's not what Struct feeds are today.
Not to mention the infinite buckets with similar topics and names.