A 51% attack would undoubtedly pose quite a serious problem for any Blockchain, regardless of the software used to access it. As a matter of fact, both Electrum and Bitcoin Core are only designed to operate on an assumption that the underlying Blockchain is secure, but that doesn't mean they provide an assurance for security of the Blockchain from external attacks.

You're right that If eventually a 51% attack were to occur right now, both of the softwares wont guarantee user's safety or be able to protect users from the potential consequences.

But looking at it from a contrary POV, even if the softwares can't guarantee user's safety, they still won't be utterly useless in a scenario of a 51% attack, because even if the Blockchain itself was compromised, these softwares could still be used to track transactions and also verify balances. These services could stil be quite valuable to users despite the fact that they don't guarantee user's safety.