and smartphones are more secure for storing Bitcoin than laptops and desktops.
This is not true, a mobile phone poses more security threat than a computer for obvious reasons, i.e. their OS. However it still depends on the user's opsec and how safely they use their device, because a desktop wallet run in an unsafe environment is more vulnerable than a mobile wallet run in a safe environment.
From this I concluded that storing seeds on paper is not always (unfortunately) the best option.
So how to you now back up your seed phrase?