I think specific features make compile time very long. Nullable types + ADTs / sum types / 'enums' are not one of them, because they don't add exponentials to

I think specific features make compile time very long. Nullable types + ADTs / sum types / 'enums' are not one of them, because they don't add exponentials to the compile time like type inference & operator overloading does for example. Also adding nullables after the fact isn't very hard, other languages such as dart & objective-c have done it. The migration path isn't that hard either, since it is a completely new syntax that doesn't have to be a breaking change, so migrations can be gradual. Out of all the features that the new higher typed languages give, I think nullables are probably the best one out of them, and one of the cheapest for all the others to adopt. And while your adding nullables, you might as well add sum types because that is how you implement it anyway.