How about this, if you didn’t have FERS you would have CSRS, which is a more costly system. No two ways about it ~ FERS PLUS the Thrift Savings Plan costs the government less than CSRS.
How about this, if you didn't have FERS you would have Social Security. SS plus TSP would cost the government less than FERS plus TSP (and be just like the private sector so nobody would complain)