Seniority is meaningless unless the employee has a union or employment contract that specifically makes seniority an important issue. There is no legal protection for an employee who devotes all of his or her working life to one employer. Even unions are starting to back off from making seniority an important factor in their contracts.
In a downsizing or layoff where money is the biggest issue, the longer an employee has been with a company, and the larger his or her salary, the more likely it is that employee will be let go, because if the employer is being pressured to reduce expenses, it is easier to terminate a few highly-paid employees than it is to actually try and make other cost-cutting moves. Once the highly-paid employee is gone, his or her salary amount is no longer considered an expense.