You simply add each device you want to login with as a trusted device. Most services allow you to add as many devices as you want. I do not understand why this is a adoption barrier.
And then if you get a new device, you have to somehow log in on that new device (with either backup codes, enabling TOTP or SMS 2FA temporarily, etc), and add the new device as an authenticator. And repeat that across every service. Same for new/existing accounts, people don't want to have to add every device to every service they use. Not being able to use your WebAuthn authenticator on other devices is an adoption barrier (as is the alternative of buying a hardware authenticator).
