Well, you use multiple keys.

If you have just one Yubikey and you lose it, then you lose your SSH access.

But that's true as well if you have just one SSH key on your computer and you lose your computer, right?

One nice thing about using a YubiKey GPG key for SSH is that I don't have to generate new keys for every device.

I have offline key on two different devices, haven't lost one yet over the last decade!