I think the thing with open source (re: your free labour point) is that it's entirely voluntary free labour - I know that wasn't the thrust of your point but there are pros and cons to it. The lead dev could one day say 'fuck it' and walk away, but for a project of any size/popularity there's a lot of people ready and willing to fork it or ask for ownership to be transferred. It's not very often a very popular bit of code is totally abandoned.

Open source, to me, offers a sort of peer review system. Most people developing open source stuff already care about code quality and privacy, contributors also do and the myriad of people using it have a core set of people who also do. That's a lot of eyes. There's also tools to diff code so its pretty easy to spot changes. And I do do that.

But I take your wider point - it all eventually comes down to trust. But that's true of legal requirements too. And also organisation behaviour. Brave for example have been caught at least 3 times doing very dodgy stuff and yet as far as I can tell they continue to grow. I don't necessarily accept that one instance of law breaking or otherwise poor behaviour is instant death for a company. If it was, G and Meta would be long gone.

All I can do is reiterate that all of us have different things that we choose to place some trust in and we all have different ways of assessing what leads us to trust. But at the end of the day, there are no cast iron guarantees.