There is an error in reasoning in this calculation:

1. As more miners move to bitcoin cash, then the difficulty rate in Bitcoin cash also will increase dramatically and will make the mining in Bitcoin cash much less profitable.
2. About the calculation that it will take 20 weeks, if the hash power reduce by 90%. That would be only true, if all 90% move in 10 minutes at once at the start of the 2016 blocks.
3. Even you are right and 50% of miners will move in 3 weeks to Bitcoin cash. But then the Bitcoin difficulty will be reduced by 50%. And then suddenly it will be for 2016 blocks very profitable to mine Bitcoin again . And Bitcoin cash will have delayed confirmation time after. And back .. and forth