With that old North Station setup, you could wait for an inbound train on the surface, or you could wait for one on the el, but there was no way to wait for both at once. So even if service was 'more frequent', it didn't do you much good.
The current setup, with the Green LIne only on the elevated, was always intended to be a temporary measure until the Superstation was ready. They had to demolish the surface stop in order to build the Superstation.
When the Superstation opens, you'll wait on a single platform for the first southbound train that arrives -- whether it's a Green Line train arriving from Lechmere, a Green Line train originating at North Station, or an Orange Line train. That's a LOT more convenient than the present setup or any past setup.
The Superstation will be fully accessible, with escalators and elevators.