youtube couldn't have existed in a pre-broadband era, even if a few were rocking T3 connections.
the question is what doesn't exist now? we gotta build it.
You're right, but people were trying really hard to make shitty video experiences over dialup in the 90s. Really hard. Broadcast.com did audio (barely), RealPlayer tried to do streaming video, etc.
The only applications I'm seeing right now that need more bandwidth are 4k video (but honestly, 1080p isn't that bad) and upstream for VC. Looking around, I don't see anything where a 5x bandwidth jump would make something suddenly viable.
Imagine for example streamed, 8K VR - meaning two individual 8k streams. Even that would only take 100-200mbps download speed, latency would be the real bottleneck and that's limited by the speed of light.
What things can a human experience that can be digitized but require continuous 1gbps to stream? I can't think of any.
It could have existed, and, in fact, did, when broadband penetration was much lower than now, and “broadband” typically referred to DSL/cable/etc. connections of 256kbps down or better, not the 25M down/3M up it has been defined as in the US since 2015, or the 4M down/1M up it was immediately prior to that.