"discussions are faster and more productive in realtime"
Real-time discussion is great if you don't have to think about what you're going to say before you say it. Unfortunately, that places a very, very low upper limit on the level of depth and consideration that a discussion can reach. Some decisions are hard enough and some situations are nuanced enough that you need to take your time with them, enough that the additional cost of making all of that time strictly synchronous can grow exponentially.
It also excludes large categories of people, including introverts and non-native speakers. I've worked with a number of people who just flat out did not or could not participate in real-time face-to-face discussions, but who could express themselves masterfully via email.
Of course, the "faster! instant! now! now! now!" crowd don't even have the patience for slower, deeper, better-reasoned discussions, which is why they shy away from any form of communication that has any more depth than a tweet or a PowerPoint slide. Slack is great for these people because they don't even have to string words together to communicate, they can just click on a reaction emoji.
Sure, some things (operations, for instance) have genuine urgency to them, and I think real-time communication is essential for situations like that. But if you're doing work that will have an impact on the span of months, quarters, or years--either major strategic business decisions, design, or project development--you're better served by thinking deeper instead of thinking faster, and deep thought is exactly what the long-form written word was invented for.