If we can recast a genesis block every 20th block as part of the routine operation of the NXT blockchain, we have accomplished something very, very special - a self limiting blockchain that grows very slowly.
A blockchain needs to be as long as longest possible fork, at least. I don't know, half a day? Definitely not 20 blocks.
1440 blocks is the limit for blockchain reorganization if I'm remembering correctly. So it would have to be just beyond there.