So, how do you reset your password when you forget it? Well, it depends.
Some apps/sites just send you a password reset email. Apps/sites like those would reset your passkey the same way: they'd send you a passkey reset email, you'd click the link in the email, and they'd let you regenerate your passkey then and there.
Some apps/sites try to do something cleverer, e.g. requiring additional factors to reset (MFA), or appointing a "trusted contact" user who can confirm your password reset, or asking "security questions" that only you know the answer to. Those apps/sites would put you through the same process to reset your passkey.
"How do I reset my password when I forget it" is an infamous balancing act between user friendliness and strict security. The "reset my passkey" problem is exactly as hard, no easier and no harder, as the "reset my password" problem.
(Of course, it's possible to have a site that has no way to reset your password, and just assumes that you'll never forget your password. Similarly, those sites could have no way to reset your passkey. In that case, the problem is as you say: there'd be no way to recover your keys if you lost access to them.)