Necessary, because if this were not true, then we’d have no reason from the premises to think that paying senior staff with stock options would undermine employee morale. And in that case, we’d have no reason from the premises to think that paying senior staff with stock options would hurt the profitability of a business.

Not necessary, because although the author does believe that anything that undermines employee morale will hurt the profitability of a business, that doesn’t commit the author to believing anything about what proportion of reductions in profitability are due to low employee morale. There could be, for example, many other things that also reduce profitability of a business, and those other things could be the usual cause of reductions in profitability.

Not necessary, because it’s possible that the business firms that pay senior staff with stock options are among the most profitable firms. Although paying their senior staff with stock options reduces the business’s profitability, it’s possible that those businesses started off with extremely high profitability.

Not necessary, because nothing in the argument is based on what would increase profitability. Although the author does believe that dramatically increasing differences in income between senior staff and employees on a fixed income would reduce profitability, that doesn’t commit the author to thinking that reducing the difference in income would “invariably” increase profitability.

Not necessary, becauase who is more “productive” is irrelevant to the reasoning. Neither the conclusion nor the premises concern the concept of employee productivity.