Beyond that, despite the repeated claims of open source advocates, there's nothing preventing people from taking the app store versions of things like WhatsApp and reverse engineering them.
So, while being open source is not the complete answer, it certainly doesn't hurt.
I don't think we have good solutions for the problem of malicious updates in general.
The only one I can think of is a trusted hypervisor that hashes memory in the guest and reports on it. And even then, how do we trust that?
So what do you do? It comes back to making sure that 'they' can only hack some of the people all the time, and all of the people some of the time. It's preventing them hacking all the people all the time I worry about.
However, open source does give users some real recourse in the event that the project moves in an undesirable direction. I don't like what's happening, I can fork it without your permission and still have access to the same development environment and build tools the original project had. I think that's important and valuable.
I can read a commented Python or BASIC program with almost no effort unless it's very sloppy. I can tell you a MISRA C or SPARK program with associated static checks is immune to entire classes of errors without analyzing the source myself. I can tell you what information flows can or can't happen in a language like SIF implementing information-flow security. I can do all of this while expending almost no effort. So, I'm much more likely to do it than if I had to decompile and reverse-engineer a binary from scratch with analyses using the tiny information in a binary.
So, every time you say that, what you're really saying is: "Anyone could do this if they spent enormous time and energy. Sort of like they could hand-compile their C code each iteration. They probably won't but I'm countering your wanting for source because in theory they could do all this with assembly with enough effort."
It's definitely not true for correctness as assembly lacks what you need to know it's correct. It's probably not true for security as correctness is a prerequisite for it. In any case, economics is important where the effort required to achieve a thing determines whether someone will likely spend that effort. In case of binary analysis, it's apparently a lot less than source analysis.