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When Baldur's Gate 3 came out, Steam's overall bandwidth consumption went from around 18 Tbps to 146 Tbps
(reddthat.com)
From Steam's self-published stats.
Baldur's Gate 3 could not be preloaded and weighed in at 125 gigabytes on disk, so when the game left Early Access at 11am US Eastern yesterday, Steam's bandwidth utilization shot up 8x over a span of 30 minutes. I know personally, I saw my download hit over 600 Mbps across a 1 Gbps fiber connection.
Kudos to the system engineers at Valve. It is mind-boggling that they have built infrastructure that robust.
Why couldn't it be preloaded?
Because it would disrupt being able to continue playing early access right upto release, I remember reading in a tweet
Personally, I would have opted for just stopping early access to preload but people have gotten angry over stupider stuff before, I guess
I mean there is no perfect option there really, you're gonna piss people off either way. I would say though locking people from playing a full price AAA game they paid for is probably more controversial than not being able to provide a preload.