A) AFAIK, there was a single game that actually required a day one patch that was 20GB or whatever, and that was the Halo Master Chief Collection. I can hardly think of any other games that actually "required" a patch that was more than a couple GBs at most. I think the only exception was Sniper Elite 3 that had a 10GB patch, but it wasn't even really "required" to play the game.
B) ACtually, pretty much all games don't actually need those day one patches to be playable, the whole point of day one patches are to fix whatever issues a game might have had that devs couldn't fix before release. For a recent example, look at the Witcher 3. When it released on the Xbox One to reviewers and such, they reported having issues with the game's framerate and AI and such, CD Projekt Red saw that and threw in a patch to help fix the issues on the official release date. Do players absolutely need to install this patch? No, not at all. But is it there if you want the game to run better? Yeah, sure (though I guess the day one patch fixed one issue and then caused another, but whatever). And it's a "small" 600MB patch, a far cry from "20gb+"
C) Maybe you're confusing disc installations and day one patches, because most current games do require you install 20-50GBs of data to the HDD which can take some time, but this is simply to help decrease load times of games and lengthen the disc drive's life.