You're supposed to have multiple devices (phone, tablet, laptop, desktop, etc) that all store your passkey securely (it can't just be sitting on the SSD...). You're also supposed to backup your passkey.

Yes, if you lose access to all your devices it could prove challenging to access GitHub... but that's a pretty unlikely scenario and most people should be willing to pay the price (what self respecting programmer travels anywhere without two or three devices?)

You are also supposed to backup everything you have on GitHub elsewhere... so it shouldn't be a total disaster if you lose access. It'd just be annoying. And presumably GitHub has some kind of recovery process for someone who's lost access to an account? What if the repository maintainer dies? Someone else has to be able to take over.

On the other hand - if my bank required a passkey... then I'd probably switch banks. If all my stuff is stolen or destroyed, I still need access to my money. And if someone compromises my bank... well it's just money. The stakes are far higher if a popular GitHub repository is compromised.