While a big proponent of this, to my mind, it seems a bit counterintuitive to place your trust in a community who will probably cannot be held into account once some bad actor slips into their ranks, creates a bad patch and empties my bank account.
It's important to note that GrapheneOS is not some niche barely-used project. It has existed since 2014 and is used by multiple hundreds of thousands of people at this point. There are also many eyes on the project through people forking it to make their own products, people maintaining their own builds etc. GrapheneOS is also reproducible in addition being open source.
On our side, we are very particular about accepting outside contributions if they don't need meet our standards, and code is heavily reviewed within our team before being merged.
I'd also recommend giving https://grapheneos.org/faq#audit a read through.
All in all, your concern, while valid, isn't something that's likely to happen precisely because we're very aware of situations where it has (see xz) and are therefore very vigilant. The kind of thing you're worried about isn't likely to come from a big project like GrapheneOS that has many eyes on it, but rather something small that's used everywhere and barely has a couple of devs working on it, if that (again, see xz).
I think of two things, the Solar Winds build corruption, and putty's mishandling of e521 keys.
What is your vulnerability to a similar disaster, exploited or not?
From what I have observed, nobody is held to account when there is a software issue, commercial or open source.
Open source software is everywhere. Do you think Microsoft or Redhat going to be held to account if they accidentally added some backdoored OSS code? Moreover all of the development happens in the open and you can build it yourself. I'm not sure what the alternative is. Just trust Apple has their shit together with iOS?
At least graphene wouldn't be expected to shield the perpetrator.