Keyword: a key. It would only take getting lucky once to get all of your money. With a wallet.dat you would need to go address by address. Who knows what those fuckers have in store when it comes to quantum computers within a couple of decades. Plus any mistakes on your part leading to key derivation exploits... yeah no thanks. Electrum seems like a bad idea for serious cold storaging.
The security of Bitcoin derives not from having many keys, but from having strong keys to begin with. The point at which randomly generated private keys become vulnerable to folk "getting lucky" we have a massive problem on our hands.
By the way, Bitcoin Core since like 0.13 I think has used deterministic wallets, so all your keys in wallet.dat are derived from a single master key now.