Topic: SDLC and Test Process Imporvement

If we consider that at each stage of the Software Development Life Cycle testing is involved - does it follow that the process improvement of testing also improves the SDLC?

Or are we doomed to improve our own corner of the world and maintain the process of interfacing with other parts of the SDLC in a defective manner?

Cannot we not say: "we continously improve our process, some communications are defective so we improve those also, so from now on work to this new interface"?