I can only speak for AA, but I know every year they draw a cutoff line for exceptions. One year I had 45,000 miles and called AA cust service, and they went ahead and gave me PLT for another year. They supervisor mentioned to me that the cutoff was 40K that year...
Also, on AA you don't get downgraded more than one level a year regardless of activity. Therefore, if you have PLT and only fly 10,000 miles (for example), you would qualify as Gold the following year even though you didn't fly the necessary 25K miles.
FYI...