I'm not sure what point you're trying to make. There are salary bands precisely because of this. Also productivity isn't the only metric. A person may get a higher salary for the same position because they bring in more experience or relevant skill set. A good chunk of more senior development work is figuring out which problems are worth solving in the first place. A junior engineer with that talent is far more valuable & senior engineers can differentiate themselves in that way as well.

As for replacements, for more senior positions its not a 2 week process. You typically have someone more interim step in if there's nobody existing on the team that can step in.