No one expects anybody to sit on their hands waiting for anything. The vehiclesizes file has primary and alternate calls for models. The modder uses whatever he wants for the primary and uses a standard ship for the alternate. No work stoppage.
If a standard is adopted later, great, the modder can patch his mod.
As for artists catering to modders, most artists want to see their stuff being used so they kinda have to work with the modders, in a partnership, for lack of a better term.