The works of artists and to what degree that work can be shared among others have been under question for a long time. Of course, the person who came up with a melody, lyrics, or another form of a mostly original piece of art should be given credit. It is true that all information and imagination are based on pieces of previous knowledge and ideas, but it is important to consider the proportion of the percentage of originality. If the majority is that of the original creator, then the work cannot be considered a new idea or piece.
A lot would depend on how the work is used. If it is for personal use or to share with a small circle of friends or relatives, then it would be acceptable. But, if it is made for the public in order to gain popularity or sell for profit, then it should be prohibited. As noted, the internet does provide a great number of videos, music files, and other types of information for the public, but people use it for their own purposes. Individual entertainment and gaining knowledge should be allowed to all people equally, as no one really owns the facts and information that exists. The boundaries set should be balanced by reason, morality, and law, so that there is no mental or physical harm done to the person who produced the work.
The affirmative right or defense is a very questionable concept. From one perspective, if the new work is mostly original, the burden of proof should lie on the prosecution. But, if it is clearly similar to the previous work, then it should be the defendant who must prove why his input is actually greater than the original. There must be at least a 70% rule, which requires only a small portion of the previous work to be used. The amount of the same melodies, images, or ideas must be limited to a concrete number so that people do not simply change a portion and claim it to be unique.