I work in the realm of public art projects and thus have collaborated with a lot of artists, mostly painters and sculpters. I must say the success of any piece of artwork - especially the ones created with the public in mind - is not the work of a single man or woman, but the result of the collaboration of a team of professionals - designers, marketers, financiers, metal bashers, crane operators, all the way down to the assistant who helped review the contracts.

So a deal is a deal, if one agrees to the terms of a deal (oral or written), it must be honoured, otherwise there will be no collaboration. Imagine the marketer who helped putting the project on every headline turns around to ask for an extra cut of the success? Not possible, but he/she can certainly charge a higher fee for the next project.

Nowadays artists are much more educated about the value of copyright and we as art consultants/project coordinators help intermediate how deals are structured, so the value of the artist's work is properly recognised and rewarded.

As others have commented, success builds upon success, one must be able to see the bigger picture to be successful in any career pursued.