The case that non-deterministic wallets actually protect anyone is slim (attacker only has access to ancient backups of the victim), and outweighed by the risk to the user that their backups will silently become out of date as their wallet's original keypool is depleted. I think everyone should forget about non-deterministic wallets. They're a historical quirk with few parallels to other systems.

I find your presumption that a Bitcoin wallet's balance should be immune to an attacker who gets a copy of it to be surprising and unfair. It's the same situation with any cryptographic keys: if an attacker steals your PGP keys, then they can use them to decrypt data or sign data for all time. If you want to protect against the situation of an attacker getting your old data, then it's up to you to rotate your keys (/wallet) and update your backups.

Proper key management is a hard problem. This is a fact that the cryptocurrency scene has been relearning for years now.