>>> How would sub-classing help in such a case?

Simple example: If you use "new StreamWriter()" all over the code as opposed to one place in your own subclass - if/when the functionality (of StreamWriter) gets changed or discontinued, you either have hundreds of places to modify or one place--in your class code.

So you don't subclass anything?

Do you believe base classes in .NET version 2.0 will work the same in .NET 5.0 or 7.0?