Whether or not a longer copyright term will help promote the arts are encouraging more investment in art production is debatable. I have a strong oppinion, but so do many others with the opposite.
However, there is no theory whatesoever that retroactively extending copyright terms does anything to promote the creation of new art/culture (recall, the whole point of government granted copyright monopoly in the first place.) In fact, there is strong evidence that works still under long copyright are supressed until they become public domain.
I think we can conclude that any politicians singing on to retroactively extend copyright terms are clearly corrupt.