Anyway you look at it, a cover letter is now a dynamic document which needs to be specifically tailored to the job/employer in question – otherwise, don’t bother.
Why turn down a generous raise, new office, extra PTO, or any number of potential perks or advancement? Because it likely won’t last. The honeymoon was already over.
How you leave a job is as important as how you do a job. That final impression out the door is that last memory your previous boss and coworkers will likely have of you.