dyga

Index is assumed to be relatively stable ( modern indices are based on Cap weighting , so their composition hardly vary over long periods).

When you use stratified sampling , you are subject to more tracking error (if individual positions move much more than the index) and so you have to continuously trade to get back to sector weight limits or max individual weight limits . Hence trading costs could increase.

Remember, a passive index fund holds diversified positions ( probably built over long period of time ) but hardly trades in or out