it's not "genetic degradation". the cavendish banana is cloned all from the same genetic line, meaning there is no "genetic degradation". the problem is fungi, bacteria, viruses, and other things that negatively affect the sustainability of the cavendish banana.

because the bananas are clones of the same line, there's no opportunity for natural selection, no mutations that might provide resistance, nothing to be carried from the heartier forefathers to the offspring. it's not genetic degradation... it's genetic obsolescence. if they were still using just selective breeding, this wouldn't be an issue.