That is why the concept of
air gap system exists and we always recommend it in Bitcoin world to those who want to create a wallet with the term
Cold Storage.
By being on a clean system that has no connection to outside world, you eliminate "contamination" risk where for example a malware could access and steal your seed phrase.
Agree that, instead of using the mnemonic as a backup, a simple backup of the wallet file like Bitcoin Core, Electrum, Bitcoin Wallet for Android do, does not reduce remote access to your wallet much more, since in these wallets we have to encrypt and then export them to external drive?
The same rules apply here too, if your system can be infected by a malware, that malware can also access the wallet file (encrypted or not) and steal the secrets inside it too.
Additionally the flaw in using the wallet
file as your only backup (aka digital storage) is that in this type of storage the data can be lost for different reasons, from physical/electrical damage to the device it is stored on (eg. a USB disk) to
data decay. This makes digital storage bad for long term storage.
This is the way. Air gapped is essential if you have any decent amount of BTC. It's liberating being your own back but like a bank, you need to take security seriously.