I think you didn't quite get the point of engineers of different levels of quality, talent and opinions working on the same monoliths.
Eventually they tear down any boundary, even those in the build system.
Developer discipline is something that eludes many companies for lack of enough high quality engineers and awareness for the problem in upper management. It's easier to quibble over formatting guidelines.
But the problem with this is it's a technical solution to a social problem.
If your developers are writing crap code in a monolith they're going to continue writing crap code in microservices but now you have new problems of deployment, observability, performance, debugging, etc etc.
As an aside I have a sneaking probably ahistorical suspicion microservices hype happened because people realised Ruby (or similar trendy dynamic languages) often ended up being a write only framework and rather than try and recover some codebase sanity people would rather abandon the code entirely and chase the new codebase high.
> If your developers are writing crap code in a monolith they're going to continue writing crap code in microservices but now you have new problems of deployment, observability, performance, debugging, etc etc.
Anecdotally I witnessed this once. There was this huge ball of mud we had that worked okay-ish. Then the architects decided "hey microservices could solve this", so we started building out microservices that became a distributed ball of mud. Every microservice shared and passed a singular data model across ~30 microservices, which made things interesting when we needed to change that model. Also, we took mediocre developers and asked them to apply rigor they didn't have in developing these services to that they were "prepared" for the failures that happen with distributed systems.
The big upside to management though was that we could farm out parts of the system to different teams on different sides of the planet and have each of them build out the microservices, with each team having different standards as to what is acceptable (what response messages look like, what coding standards should be, ect). All of this was less of a technical problem and more of a managment one, but we felt the pain of it as it was made into a technical problem.
I know. Which is why I have architectural tests that scream if someone uses the wrong module from the wrong place (adds a dependency that isn't what we want).
Of course, any dev could just remove the test, and by that tear down that boundary too. But that test being removed is much more obvious to the code reviewer who would not have noticed the dependency being snuck in. The architectural tests can also contain comments that form a high level documentation of the architecture. Unlike "architectural documents" in the form of some up front word document, this type tends to stay up to date.
In .NET I do it om the binaries, I reflect over the assemblies and list the dependencies.
In the same run one can also validate that there are no incompatible transitive dependencies.
Within assemblies (between namespaces) is much harder unfortunately. That means assemblies have to be made to make module boundaries obvious, even though it’s a bit of an antipattern. There are tools such as nsdepcop that watch namespace dependencies, but it’s brittle and a lot of work.
So your argument is that bad developers will mess things up. I am sure most would agree with that argument. What does that have to do with monolith vs. micro-services? Bad developers will make a mess of micro-services as well.
You most definitely can enforce boundaries in libraries. Simple make sure that each library can compile and work with only the dependencies available it is allowed to have.
Code review is usually a bad place to catch design flaws, unless it's done early. Usually a bad design means re-doing a lot of the work. That means either getting management buy in or not hitting expected velocity. If not communicated well it can also lead to bad feelings between the writer and reviewer.
Where I work the code review process has mostly broken down. There is just no bandwidth to get anything done beyond the most basic sanity checks. To actually make someone improve their code I'd need the time, energy, authority, good will and probably other things to explain and teach the other programmer what and how to do it differently. But shit needs to get done and if the code works it's hard to convince management why I should invest so much of our time essentially redoing work that doesn't need redoing.
I suspect that this bandwidth problem exists elsewhere also.
Eventually they tear down any boundary, even those in the build system.
Developer discipline is something that eludes many companies for lack of enough high quality engineers and awareness for the problem in upper management. It's easier to quibble over formatting guidelines.