Sorry for a late reply.
I disagree. It's still fairly easy to check input's disabled property, you can use either the attr() method of Navigator or the @ syntax, which is redefined for Navigator as attribute access operator:
The `isDisabled()` method has been moved to FormElement module class as you've noted because the boolean value it returns is semantically correct only for buttons, inputs, options, selects and textareas. I believe that the move leads to a cleaner API and therefore the minimal overhead now required is justified.