I guess nearly no one will safe the Nsec like a Seephrase.
If i would do so i must now grab my hardware wallet / signer to sign this message. Who would do this? 

If you would lost your nsec a other person can steal your identity. But i think it´ll ne easier to get your identity back as it would be to get your sats back. You can just grab a new nsec and kind of rebuild your identity, but your sats will be gone forever. Or at least someone else will hold them.

But the technic is already existing:
https://shop.lnbits.com/product/remote-nostr-signing-device

And in the feature it may be neccesary for companys or politions.

But i think we´ll become a technic to do this with our phones. For example with our biometric functions.