But isn´t it then like a hot wallet? 
I mean you use it on your pc or phone online.
But to be honest i don´t really have a deep knowledge of how nostr works in the background. But for example primal could see my nsec, or not?

You´re right because of the identity. But it would be way easier to proof that your old npub isn´t under your control anymore and your new npub is yours. I mean if you meet your friends in person you can just tell them that your nsec was stolen and didn´t belong to you anymore. It would still suck, but with bitcoin you couldn´t proof that the sats are yours if your seeds are stolen. So your sats will be gone forever.

And on the other hand you´ll use your seeds, or your hardware wallet maybe once a month or something like that, because you´ll use a second bitcoin layer for your daily use. On nostr even know i sync messages, don´t know, 20-30 times a day. In a few years i guess i´ll sign even way more. So it wouldn´t be useful to sign it with a hardware wallet every time.