Since the ICJAG has been mentioned, I extracted the following from their guidelines for judging motorcycles:
promote the proper preservation and correct restoration of motorcycles
deductions will be made for over-restoration
Motorcycles are meant to be ridden
I highlighted the terms that are totally subjective and thus subject to disagreement now and certainly subject to change with changing fashions in the future.
There's a tendency for people to believe that we've arrived at current judging standards by a process of eliminating incorrect ones and thus that we're closer to "true" standards today than at any time in the past. But, this isn't any more true than it is in other areas of fashion. There's no reason to expect today's clothing styles to be any more permanent than were Nehru jackets, bell bottoms, or paisley ties. Not very long ago AMCA standards rewarded too much chrome, Imron paint, and polished cases, so what is subjectively believed to be "proper preservation," "correct restoration," or "over-restoration" in 2018 is unlikely to be identical to how those statements are interpreted in in 2028.
The "motorcycles are meant to be ridden" statement is no less subjective than the others. An alternative statement just as well could be "motorcycles were meant to be sold for a profit," as evidenced by the fact when that no longer was possible the manufacturer went bankrupt whether or not the motorcycles they previously made were ridden. The designer of a concept bike may well have not meant for it ever to be ridden, only to be displayed. If gasoline costs the equivalent of $500/gallon in 50 years people may still like to look at them at shows but insisting they be able to start would be an unrealistic standard to apply. The point being, ICJAG's third statement above is completely subjective and subject to change just as are the other statements.
However, despite the ever-changing standards, a self-styled organization like the ICJAG is useful because it helps ensure everyone who enjoys preparing bikes to "concours standard" today is judged against the same set of expectations. And, if you don't like the standards an organization like ICJAG (or AMCA) uses in 2018 then wait a decade because history tells us they will change as the attitudes of motorcycle owners change and as new people with different opinions replace old ones as judges.