Require frequent passwords, humans cheat, boom: your brute-force space just went from 1024 bits to 14, assuming you can onboard a red-team plant far enough to get the template for the default passwords.

If I know _bigcorp_ gives defaulted credentials in the format of [First Initial + Middle Initial + month_day] then not only can I piggyback a trivially-created IT/support ticket, I can also just guess that in 60, 90, 120 days, your credentials are the same, but the month_day - even if not correct, the search space is reduced by magnitudes.