A number of clubs and societies already do this to some extent, think of EXPONG or society shows/meets 009 Society, NMRA. There are also scale orientated shows EXPOEM, N guage show, etc. As others have said there are Freemo or NTRAK modular meets. These however tend to cater for niches within the hobby.
As an exhibition manager of a general model railway show, and previously involved with an American only meet, it's quite clear that specialist shows / meets don't attract the general public in the way a general show does. Let's also be quite clear that low numbers of "hard core" hobbyists through the door doesn't make for an economically viable show - halls cost money to hire, layout operators need overnight accommodation and petrol money, catering needs to be paid for, etc .
All the feedback we have received from surveying visitors indicate that the public want to see entertaining models they can relate to, regardless of prototype fidelity.
I certainly don't see specialist shows becoming the norm any time soon especially as exhibition costs continue to rise, but they will certainly be part of the scene.