Cost
Well as the chosen design appears to have a glass walled ground floor and solid walled upper floor with three boxes - one straight, one kinked and one more kinked , linked by glass wedges I doubt that an arched roofed oblong would be much more costly?
Air handling
The working station would be in a separate section with glass walls to the museum, and would be/could be more open at the end and overhead. The museum public would still be close to the working GCR and vice versa.
Inspiring
Yes, I agree, large empty spaces can be inspiring - but it will not be empty and from the outside will look like ...a warehouse or cold store - hardly a design icon.
I am not an expert in these matters, but merely an interested observer and supporter of steam and the GCR.