Very surprised they didn’t do preorders for this

They get extra publicity for crashing Steam. That's quite an achievement these days!

I was thinking about this for the recent GMTK game jam, which crashed itch.io.

Is it really such a bad thing for your launch/event to crash a platform? Nobody is going to decide not to buy Silksong after all because it's so wildly popular it brought down Steam. It generates a great deal of positive headlines. To me it seems like a good problem to have.

1. That's Valve's prerogative, not the developer's. If they're not partnered with Valve they may not have had the option to enact pre-purchases. [1]

2. The game has no DRM and Steam preorders (in my experience) download the game files so people can play instantly on launch day. (They call it 'preloading'). For a game as highly anticipated as this, it'd likely just be cracked, leaked and pirated the moment preorders came live.

[1]: https://partner.steamgames.com/doc/store/prepurchase

You don't believe Team Cherry's prior sales of Hollow Knight would reach the bar noted in the docs you linked?

Aside, your second point is incorrect. The SteamDB folks have a public write up on analysis of the preload system: https://steamdb.info/blog/steam-download-system/

#2 seems like a technically solvable problem: AES-encrypt the preload and distribute the key on launch day.

I thought they already did something like this. I recall some people complaining because the naive decryption process would double the required hard drive space while the bundle was unpacking on release day.

There is no need for the decryption to double the needed size. You can chunk it to something reasonable and effectively rewrite the file in place.

Pretty sure they already do it. I've also noticed that if you have a fast connection (like 1 Gb/s down), it might be faster to download the game when it releases than it is to preload it and then decrypt it, since the decryption process is quite slow.

Of course they do. GGP has no idea what they are talking about.

They could've enabled pre-orders on Nintendo and Xbox