Interesting question.
My DI told me it was a 5.56mm, magazine fed, gas operated, air-cooled, shoulder fired, weapon.
Based on current mainstream definitions I think it would be an AR or an AR variant.
The weapon system was designed an built off of the current AR platform, but I see your point -- as the platform evolves with new improvements there would be a point when it could morph into a completely new system. But that point would be somewhat of a subjective assessment.
In a basic sense the criteria could consist of a percentage of parts interchangeability, and magazine compatibility.
Maybe a similar argument could be made for current production 1911 pistols, lots variants, and lot of current factory installed modifications from JMB original design. But I think that is one of the attractions to the AR platform, it can be easily tweaked and modified to fit the users desires, and still have the same basic functional operation and ergonomics.
By the way, it took me years out of the service to stop calling my semi auto AR15A2 a M16, so if we change the name now . . .I am never going to keep up.