It doesn't matter what roles any employee previously filled—we're only concerned with average seniority between writers and programmers, and this doesn't help us to evaluate that.

This is exactly what we need to evaluate the argument. If the average writer's seniority properly results in 20% more pay than the average programmer, the executive’s argument is strengthened. However if this isn't the case, the executive's argument that the disparity is acceptable based on seniority falls apart.

The argument is just about whether the average difference in salary and benefits between writers and programmers is justified. The type of benefits that workers receive doesn't help us evaluate that; it's just not relevant.

This can't help us evaluate the argument because it wouldn't provide any information about average seniority. All this does is suggest an ad hominem attack, which is a logical fallacy, not a weakening strategy.

We’re not comparing between executives and programmers, we’re comparing between programmers and writers. The executive's salary doesn't help us to evaluate whether the disparity between writers and programmers is justified.