Trying to hook everything into the build management system is a source of technical debt, your using a tool for something it wasn’t designed.

I never said to do this..

CI tools have different DSL and usually provide a means to manage environments. Certain integration and system level tests are best performed there.

Hm? no, definitely don't do this. We've literally spent half a decade at my current company trying to get rid of the system that previous devs that thought this was a good idea, and this is exactly what they did. Your integration tests and system level tests should not depend on the environment you run them in. You should be able to execute them from anywhere and have them run the same. Depending on CI to do that for you means that you are tightly tied to not only your CI, but whomever maintains that infrastructure, the resources around that infrastructure, whether those are build machines, secrets, or even the workflows themselves.

For instance I keep system tests as a seperate managed project. The project can be executed from developer machines for local builds but I also create a small build pipeline to build the project, deploy it and run the system tests against it triggered by pull requests.

That... has nothing to do with CI/CD.. You're just not using your build management tool (which is built to execute tests) to execute your tests...

This is why I say the build management system doesn’t really change, because you should treat everything as descrete standalone components.

I do not understand what you're saying here. You're going to have to explain more.

The Parent POM gets updates once every six months, the basic build verification CI pipeline only changes to the latest language release, etc…

And is that because you're not actually updating your dependencies that are in your parent POM? Because we update dependencies multiple times a week. Not really sure where you're going with this though.

Projects which try to embed gitflow into a pom or integrate CD into the gradle file are the unbuildable messes I get asked to fix.

I don't think I ever recommended either of those things. For one, gitflow is a terrible workflow, and two, why would you need to integrate CD into the gradle file? Your pipeline calls your gradle tasks to perform the things your build needs. Not making gradle into a CI tool.