People in their 70's and 80's aren't known for their stamina and mental prowess.
The WT needs folks who aren't settled in their ways and are willing to mold with the new changes coming from Bethel HQ.
The older folks in full-time service to the "Order" are covered by WT health self-insurance. Therefore by getting rid of DO's and older CO's there's an immediate cut in expenditures for these folks to get care at the expense of the WT.
The COBE position is just more workable in the hands of someone who has a quicker mind and can receive and respond to matters from WT representatives and follow through on instructions. A person in their 80's may have seen too much and know too much so it's better for the WT to sideline them.