But would there be the passengers numbers to justify a train service over a bus? At present - NO
That's the sticking point. Belfast International does not currently have the passenger numbers to justify the expense of a rail link.
Both Belfast airports combined serve a small population and for the size of population they do serve the route choice isn't too bad. There's direct flights to two of the largest hubs in the world (Heathrow and Amsterdam) and a decent selection of other destinations.
The crux of the argument in regard to routes is will they pay for the airline. You could have the newest and most shiny airport in the world, that alone does not attract routes and airlines, you need a business case for those routes and passengers to fill the planes.
The comparisons with Dublin airport are highly flawed for many reasons. Dublin serves a purpose and I use it a lot, but I also use the Belfast airports a lot. It's not a case of one or the other.
We also have to remember that Dublin Airport is publicly owned and benefited from billions of taxpayer funds to upgrade facilities, both Belfast airports are privately owned and need income in order to justify investment.