Im also worried that a smart enough individual could somehow derivate private keys from owning a couple of private keys + the master public key (so they could have access to ALL private keys generated thereon)
That's only applicable for non-hardened keys. For hardened keys, they aren't susceptible to this because they don't even have master public keys in the first place. Now first of all, you shouldn't be sharing any of your private keys and if they have access to one, they would have access to the master private key.
I was wondering: Isn't the ideal design to be that you would need to backup your wallet every single time you create a new address? so if an attacker managed to steal your wallet.dat, that is all he would get, he couldn't sit and let you store BTC indefinitely until he steals all of it (waiting either for 100, 1000 or infinite addresses in the case of HD), he would only get whatever your wallet.dat had at the time he stole it.
If this makes no sense let me know and I will try to rephrase my point, it's just an idea, and would like further discussions on either what's safer, classic wallet.dat format or new BIP32 HD format.
It isn't the ideal design by any standards. If you had to backup your wallet every single time, most probably won't bother changing addresses and that results in the problem with address reuse. You would guess that most probably wouldn't bother backing up it every single time and more coins would be lost.
Hardware wallet devices aren't really to be trusted in my opinion. A general purpose laptop with Linux in an airgapped way to store the private keys is the best way I can think off. If you create a brand new wallet in an airgapped Linux laptop, I guess even if it's HD, it wouldn't matter, the chances that it gets hacked are really low. Paperwallets suck, you need to deal with several addresses, so you would need several papers all over the place, not very convenient.
As opposed to hardware wallets being insecure, I would say that its just too expensive, as compared to a Raspberry Pi. Most of the firmware are audited and open sourced so it can be trusted, if you know what you're doing.
The new HD wallets are definitely superior as compared to the old wallet.dat. The hardened key derivation for Bitcoin Core doesn't allow others to use the master public key which eliminates that attack vector. With the old wallet.dat, you are supposed to backup every 100/1000 transaction which is what many failed to do and lost loads of money due to that. The new HD wallet.dat basically only requires 1 backup, unless you encrypt/change password. You shouldn't be worried about getting hacked. There would be more people losing money by forgetting to backup and hacking as compared to HD wallets. Hackers aren't all that interested in
potential long term profit.