Consider the binding space, including whether BinderBase<T>, IValueSource, and IValueDescriptor are needed in the public API for usage beyond SetHandler

[jonsequitur]

namespace System.CommandLine.Binding
{ 
    // This is the only hit of BinderBase in this proposal.  Is it needed?
    // The "Base" suffix is recommended against unless it's a very niche carve-out from a class that uses the better name.
    // Basically, if more than 30 people in the whole would would ever need to write it in their code you need a better name.
    public abstract class BinderBase<T> : IValueDescriptor<T>, IValueDescriptor, IValueSource 
    {
        protected T GetBoundValue(BindingContext bindingContext);
    }

    // In general, interfaces should only define capabilities, not actual structure.
    // If at all possible, this should be turned into an abstract class.
    // Note that interfaces can basically never change (we have never successfully added a DIM as a way to expan an interface).
    // So with this interface there's no proven way to add, e.g. T GetDefaultValue<T>(Func<T> defaultValue) in a later version
    // in a way that avoids boxing values into object.
    public interface IValueDescriptor 
    {
        bool HasDefaultValue { get; }
        string ValueName { get; }
        Type ValueType { get; }
        object GetDefaultValue();
    }
 
    // Interfaces with no members are effectively prohibited.
    public interface IValueDescriptor<out T> : IValueDescriptor 
    {
    }
 
    // This doesn't look like a capability, so a base class is very strongly preferred over an interface.
    // (We really don't like interfaces)
    public interface IValueSource 
    {
        // We really don't like ref.  Is this actually C#'s `out`?
        bool TryGetValue(IValueDescriptor valueDescriptor, BindingContext bindingContext, ref object& boundValue);
    }