"Maintaining an unusually large number of investment options on a plan menu will not absolve a fiduciary of its failure to continually monitor and remove or replace poor-performing, high-cost or otherwise imprudent investments from the menu.... Adherence to processes and procedures governing the construction and maintenance of the plan investment lineup and diligent documentation of how those processes have been followed may warrant some judicial deference.... There may be a potential chilling effect on the ever-expanding market of retirement plan investment options."