I waited hours to play Witcher 3, Cuberpunk, Assassins Creed Odyssey. With WoW I was playing within minutes.
I worked on Guild Wars 2, which has this feature. I made a first prototype of it that streamed all content on-the-fly. It's pretty easy to implement - you have an abstraction that asynchronously loads a file off of the disk, and you can just make that download from the network instead.
The tricky part is when you want to ensure all the assets are there for a specific area before you load in, or simply knowing what order to download things in. For example, there was a starter area of Guild Wars 2 that spawned monsters from many other areas, this meant that the manifest of what was needed was enormous for that area.
So the 'playability' threshold becomes a trade-off between game experience (assets popping in as you play) and quick entry.
If you include download time, even that doesn't hold up to your claim.
This is not an uncommon internet download speed even today (let alone 5 years ago when it launched) according to the steam download stats (it varies by country, check australia for example, US is a little over 100 average): https://store.steampowered.com/stats/content/
Even at 100Mb, 45GB is 1 hour. Witcher 3 looks to be about 30GB but Cyberpunk is apparently 70-100GB.
Do the math next time :)