It takes a lot of development time to produce hard core sims. The way that most of the software developers are going about it at present makes it virtually impossible to produce a hard core sim with multiple aircraft, because they seem to basically start over with each new sim. The way to do it would be to take a simulation engine and design a game for expandability in the way Microsoft have done with Flight Simulator.
For example, Jane's could take the engine for F/A-18E and build an F-4B/C sim using it, which would take (optimistically) one year. Six months later, they could release an F-8 add-on. Six months after that they could release an F-105 add-on, following with South Vietnam terrain and the F-100, etc. Eventually they could upgrade the engine, but making it compatible with the previous add-ons, as was often the case with Flight Simulator.
The main problem with this as I see it is that by the time the first add-on comes out, everyone will be playing the next sim from some other company, and by the time the next ones arrive most users will regard the graphics as too inferior for them to contine using the product.
If a developer can think of a way to design a simulation using a modular design, allowing 3d models, terrain, textures, AI, etc. to be upgraded while remaining compatible with everything else then it could work. It sounds like an extraordinarily difficult task to me, however.