This is akin to what Marriott does. If you've been Plat for X number of years (I think it's three) if you don't quite make it, they extend you for one more year. They also have a soft landing, so someone would usually go from Plat to Gold even if you've ony qualified for Silver.

While your idea makes sense, how much would DL lose if people didn't have to pay for otherwise unflown flights for mileage runs?