Using the actions of other law firms in the region as instructive, it is not unlikely that the firm does not believe that it presently requires a designated employee for the work you describe. Very likely the work you did will be absorbed by another employee or employees. Most businesses, including law firms, have a working idea of the revenue attributable to the various position functions. Most likely, the fiscal contribution from your position -- even though done well -- was not sufficient to cover the overhead of an employee in that position.
It is not clear why you believe that salary cannot be a criteria used by the employer in making lay-off decisions. That is not the state of the law.
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You have been misinformed. Employers can and often do use salary as a basis for laying people off to cut costs. There is nothing unlawful about that. In some cases, an argument can be made that laying off the highest paid workers creates, what we call, a disparate impact on older workers and may be the basis for an age discrimination claim. But in the rare case where this may work, it usually applies to large scale layoffs not individual cases.
They say you get what you pay for, and this response is free, so take it for what it is worth. This is my opinion based on very limited information. My opinion should not be taken as legal advice. For true advice, we would require a confidential consultation where I would ask you questions and get your complete story. This is a public forum, so remember, nothing here is confidential. Nor am I your attorney. I do not know who you are and you have not hired me to provide any legal service. To do so would require us to meet and sign written retainer agreement. My responses are intended for general information only.