Fairly true, however, the lack of direct train connection to LTN Airport for passenger originating from the Virgin West Coast Network (northern of the airport) makes things much more difficult.
Virgin Trains passenger are, in fact, routed via Milton Keynes Central and than coached to Luton Airport on a Virgin Train Shuttle calling at many places here in and there (many more than the sister service by Natexpress. Train tickets rarely offer a routing via Luton Apt P'way coming from the north.
I agree, in principle, that the service may be successful though. We'll see in the coming months!