Hi Lapon. Agreed about the type of experience. As this discussion concerns airlines, it's airline experience.
Valid points have been raised above. The ability to move airlines and not have to start at the bottom again has several advantages for pilots. Not only changing jobs for reasons such as geographic relocation but for better terms and conditions, which in turn forces the airlines to up their game in that regard.
Having key staff effectively stuck in an airline due to high seniority or simply by the prospect of losing a command if you change jobs is a HR managers dream.
HR also nowadays have more influence on Ops, and have determined that it's cheaper to hire DECs than it is to upgrade FOs.
If seniority was global, and derived from experience, the system would still serve its function, but by enabling free movement it would not hobble those it seeks to serve.