If ever a wallet would be open-source, it can be used by other people to duplicate that wallet and pretend to be an original for their thefts and attacks.
I mean yes, if code is open source then someone else can modify it and release their own version, but that is not a reason not to use open source software. That's simply a reason to verify that the open source software you are using is indeed the legitimate version and not a fake copy. It is perfectly possible for malicious users to also create fake versions of closed source software and wallets as well, and this happens regularly. All they need to do is copy the layout, design, GUI, and the vast majority of users would not be able to tell the difference. At least with open source software you (should) be able to build it yourself from the code or at least verify that the code is unchanged.
It would be ridiculous to have nor trust an open-sourced wallet as you would be vulnerable in many aspects, including your holdings to be exact.
This is nonsense. The whole of bitcoin is built on open source code. Bitcoin Core is open source. Saying that all open source wallets are a risk to your coins is just plain wrong. The vast majority of coin thefts have occurred from closed source web and exchange wallets.