> In a multi-repo world, I control the repo for my own service. For a business-critical service in maintenance mode (with no active feature development), there's no reason for me to upgrade the dependencies. Code changes are the #1 cause of incidents; why fix something that isn't broken?

This is now the “what is code rot?” discussion, which is an incredibly deep and nuanced discussion and I’m not going to do it justice here.

Just to pick an example—if you have an old enough version of your SSL library, it won’t be able to connect to modern web servers, depending on the configuration (no algorithms in common). If you have old database software, maybe it won’t work with the centralized backup service you have. If your software is stuck on 32-bit, maybe you run out of RAM, or maybe the vendors stop supporting the hardware you need. If you need old development tools to build your software, maybe the developers won’t be able to make changes in the future when they actually become necessary. What if your code only builds with Visual Studio 6.0, and you can’t find a copy, and you need to fix a defect now?

As much as I like the idea of building software once and then running the same version for an eternity, I prefer the idea of updating dependencies and spending some more time on maintenance. I advocate for a certain minimum amount of code churn. If the code churn falls too low, you end up with getting blind-sided by problems and don’t have any expertise to deal with it. My personal experience is that the amount of time you spend on maintenance with changing dependencies doesn’t have to be burdensome, but it’s project-dependent. Some libraries you depend on will cause headaches when you upgrade, some won’t. Good developers know how to vet dependencies.

If you really need an old version of a dependency, you can always vendor an old version of it.

If you can’t afford to put developers on maintenance work and make changes to old projects, then maybe those projects should move from maintenance to sunset.

> that there exists a team or group of volunteers to maintain such a build system across the entire repo

Bazel doesn’t require a whole team. My personal experience is that a lot of teams end up with one or two people who know the build system and know how to keep the CI running, and I don’t think this changes much with Bazel.

Bazel is actually very good at isolating build failures. You can do all sorts of things that break the build and it will only affect certain targets. It is better at this than a lot of other tools.

> underlying technical problem of low-signal PRs

I honestly don’t see a few low-signal PRs as a problem. PRs are not there to provide “signal”, they are just there to logically group changes.

The kind of PRs that I see go by in monorepos are things like “library X has deprecated interface Y, and the maintainers of X are migrating Y to Z so they can remove Y later”. Maybe your experience is different.

I do think that owners should not feel that they need to carefully review every PR that touches the code that they own. This is, IMO, its own anti-pattern—your developers should build processes that they trust, monitor the rate at which defects get deployed to production, and address systemic problems that lead to production outages. Carefully reviewing each PR only makes sense in certain contexts.

If you’re working in some environment where you do need that kind of scrutiny for every change you make, then you are probably also in an enviroment where you need to apply that scrutiny to dependencies. Maybe that means your code has fewer dependencies and relies more on the stdlib.