I listed many reasons why open sourcing wouldn’t make sense and you seem to have hyperfixated on just one.
Yes - Google chose a monorepo because of the engineering and business advantages it gave them. For example, all binaries had a common dependency that implemented all sorts of common functionality they found binaries on their servers needed out of the box - that’s why a simple c++ hello world on Google3 results in a many many mib binary that includes an http server for performance monitoring. It’s an opinionated codebase but the opinions are largely documented and explained and aren’t crazy (just because you may make other decisions or prioritize things differently doesn’t mean Google’s choice are wrong). And again, please don’t hyper focus on what I wrote as “oh it only has this one advantage”. There’s lots of advantages and I’m just giving very basic examples. It’s been a fairly long time since I worked at Google and someone who works there is going to have a more authoritative explanation of how things work now as things will inevitably changed some.
You may say they dug their own grave while the internal teams find it helps them run services and maintain things because there’s 1 way of interacting with the artifacts of lots of different teams. And they weren’t going to open source EOL projects anyway and maintenance decisions have nothing to do with google3 and are product and engineering led decisions.
Also, they have a bunch of tooling to manage open source projects that mirror google3 projects. It’s doable for projects that care / who it’s important to but it is also a lot of work (eg can write rules that bidirectionally rename files, functions, c++ namespaces, comments that can strip out code before outgoing synchronization, etc etc). if I recall correctly the tool itself was open sourced too.
As for customer trust, again I point out that not only is the Google3 aspect a minor detail, open sourcing wouldn’t even impact customer trust in a large way. It might with a certain segment of the technical population, but that’s not the people Google cares about in terms of public trust. And any open sourcing also has to find motivated maintainers who are not going to be motivated by 0 pay and a huge amount of bug reports and feature requests from users who started with maintenance support of paid engineers and focused product vision. Make sure not to extrapolate your own predilections as a tech engineer onto the general population.