Hope that the program continues ahead. The plane is impressive up close.
For an operator, the fact that there is not much in current orders is not a reflection of the potential for the system.
Total fuel saving to get to the launch conditions for a vertical launch is around 30-60% of the total fuel to LEO, so a 250T total launch weight is equivalent to about a 10T payload and a 500T launch vehicle, about 2/3rds the size of an Ariane 5. The fuel is not a high cost, but the structure to carry it is expended in most cases and is a cost. The effect is to remove at least 1/2 of the costs of the first stage. DeltaV saving for the design is modest but that is a log law function so gains are considerable. The greatest improvement would be in the potential for reusable stages of the vehicle, and the increase in safety, lifting 750T from zero at sea level is a much bigger deal than launching 250T at 40,000'.
The size of the final staged payload is not constrained that much, the geometry permits a Delta or Ariane style large diameter payload to sit in front of the centre section. Cost is the main driver, but there is a safety case to be made as well.
Would be a shame to see it not work for its living. It's not too far removed from Branson's program, and the SSC, would look good in Virgin or NASA colours.