Tons of people said this early on, but the people who make decisions for team tooling/workflows apparently didn't think that was as important. Symptomatic of the disconnect between what most engineers think of as "quality" versus others.
At that time, every decent mail client was also a newsreader, so people could easily flip back and forth between private correspondence and public discussion. News lets you write substantial, email-sized posts and replies, but it also plays perfectly well with quick one-liners. It allows threaded discussion, and newsreaders give you simple but effective tools to navigate it. It's fine if threads blow up and die out in a day, or if they live on for weeks. You can cross-post where a discussion touches different areas. You can post-and-mail if you want to attract a specific person's attention. It's easy to index (although i don't think we did that).
It was amazing.
This is how I use Slack as well. It works nicely for me personally.
Also text = async. Use the phone when sync is needed.
My problem is that all these networks are proprietary and isolated. When someone comes up with a really bad idea in their UI, you're SOL until they decide to fix it. When someone comes up with a really good idea, you have to hope their competitors re-implement it. And all they can do is gently try to guide their network into the place they want to occupy on the sync-async spectrum via features.
Looking back at the history of computing, I see that when a protocol is open and has many competing implementations, it lives on far beyond what the original designers ever intended. When a protocol is proprietary and implemented only by a single proprietary UI, it has a fairly short lifespan. What I can't tell is if these companies don't know their history, or if they think they're going to be the first ones ever to beat this trend, or if they don't care and just want to grab money while they can.