Morally it's the employers to blame, but the world is not built on morality, but on power dynamics.
For loyalty to be worth something, the relationship has to be at least somewhat equitable. Each party has to have the power to hurt the other significantly. If this is not the case, then the vulnerable party's loyalty is not worth much of the invulnerable party. I don't need the loyalty of the hamster I keep in the cage, what are they going to do, leave me for another owner? But I do need the loyalty of my free roaming cat, as if I'm treating them wrong, they are going to leave me one day, and so I'm short of a dear companion. I think that workers are vulnerable past and present, and so, I don't think that their loyalty is "no longer" paying - I think it was never paying. Or I should rather say "rewarded" instead of "paying".
Second thing that lowers the value of loyalty is that people don't change easily. Some easier than others, but change in general is deeply stressful, when talking about change in personal things like work, or home. So, often, and at least in part, a person staying in a job or at a specific rank is not loyalty, but resistance to change. And as such, it's not that it's not paying for the employee, it's actually lowering the employee's value, and so, their compensation too.