Agreed, but unless you have written all the code yourself then this is hard to do. We have to be able to black-box things in order to manage complexity and make progress quickly. For this SOLID helps but if it is applied a too fine a granularity then it can make things worse (and I think micro testing things with mocks etc can cause this)
"There is no theoretical reason that anything is hard to change about software. If you pick any one aspect of software then you can make it easy to change, but we don’t know how to make everything easy to change. Making something easy to change makes the overall system a little more complex, and making everything easy to change makes the entire system very complex. Complexity is what makes software hard to change. That, and duplication."