In COLLADA 1.5 fx_sampler2D_type is derived from fx_sampler_type. fx_sampler_type has a child element <instance_image> which is of type instance_image_type. As fx_sampler2D_type inherits fx_sampler_type via xs:restriction, it does NOT inherit <instance_image> as child. If inheritance would be done via xs:extension the child would be inherited.
<instance_image url="#image-id" />
Altova XML spy gives the following validation error:
The content of complex type definition 'fx_sampler2D_type' is 'empty'. This forbids any content for element <sampler2D>.
Error location: COLLADA / library_effects / effect / profile_COMMON / newparam / sampler2D
cvc-complex-type.2.1: The content of complex type definition 'fx_sampler2D_type' is 'empty'. This forbids any content for element <sampler2D>.
cvc-elt.5.2.1: The element <sampler2D> is not valid with respect to the actual type definition 'fx_sampler2D_type'.
Moreover instance_image_type inherits instance_with_extra_type via xs:restriction, too. That means instance_image_type does not inherit the child element of instance_with_extra_type.