Slightly more than half of my working days in my 19-year career have been remote. In my experience, Slack is hands-down the best tool for bridging onsite/remote. I can choose to engage with it in real-time for active participation for inevitable unplanned discussions, but there's also a historical record to reference. If the real-time chat merits "let's hop on a call" it happens then and there (in which case someone needs to be scribe, to summarize the takeaways in-channel, for posterity). Catching up w/ the conversation after a heads-down stretch (or vacation, whatever) is straightforward. Proper use of channels is important (ie, not just a firehose in #general) but very easy to establish, and the benefits compound.

It's not just real-time, it's real-time plus async/archival. Best of both worlds if done right.

To borrow your "cavemen" analogy: email and undocumented verbal conversations strike me as prehistoric / uncivilized, in comparison.

(That's leaving aside the benefits of having a single hub for ~everything, which Slack is ~uniquely suited for given all the integrations.)