If you are bad at the storing and keeping your secret phrase than should be alarming for you. Even though the hardware wallets are offline, what do you do if your hardware wallet shows a mechanical error for some reason?  The simple answer is that you import your secret passphrases into another hardware wallet.  But what if your saved offline phrases are lost?  Then you lose both and there is no way to recover it.  But if you think that you have two different qualities, maybe one is damaged due to mechanical reasons but the other is not damaged, then you have some amount of funds saved.  I think that's what your friend's younger brother is trying to say.

I would say that if you fear anything like that, it is nothing but your own negligence.  If you are so careless then you can careless about offline hardware wallet everywhere. I believe that if you're confident enough with handling with your staff there's no other options good for hardware wallet. Storing multiple wallet can become hassle. So it's best to keep only one .