Employee tries to quit after working 80 hours for 3 months straight, boss hits them with "no": 'My salary has actually declined since over the past 5 years'

Once you've told an employer that you're trying to leave, want to leave, or are going to leave, you're risking that they fire you on the spot. Even if they don't fire you on the spot, you run the risk that they're going to try to replace you as soon as possible in order to mitigate "risk to the business," stringing you along with counteroffers and promises to encourage you to stay when they know that they're going to send you packing as soon as they have a better option. See, in a lot of employers' eyes, once you tell them that you want to leave, they're no longer going to trust you, no matter what the relationship dynamic was previously. Some take the notion as a betrayal at worst or, at best, label you as not being a "team player."

You know it's over for you, too, when you start being given suspicious tasks like suddenly writing out manuals of what your role consists of and documenting your workflow for continuity purposes. Which is really your employer admitting that they have no idea what you do, but they know it's important. 

For these reasons, it's always best to stick with a decision to leave once you've made it. Even if your employer comes back with a generous counteroffer, it remains a fact that if it wasn't worth it to them to make it to you before you, so you should be wary of the fact that they're making it now. In this situation, their boss begged them to stay, making promises to them that had been made but never been fulfilled previously.