I had limited contact with my daughter as well..when they are using, they really have nothing to offer a relationship and their world gets so small and yucky, there isn't alot to connect on..I too feared she would die and that's why I didn't go no contact..I had a friend that did and her sister DID die.
It's ok to not want to delve into that world with them..of course you mourn how you wish the relationship could be..and one day it may be what you hope.But protecting your serenity is your #1 priority just like protecting sobriety may one day be your sons..