The novel Frankenstein was subtitled 'The Modern Prometheus', so it is clear from the beginning that Shelley saw that Dr Frankenstein would be eternally punished for taking science beyond the boundaries of morality. As already noted, we are on a relentless path to defeat illness and death, but there still remain many questions as to how far we can go and how far we should go: consider the debates into stem cell research and the growth of human organs in animals, to name two 'methods' of disease control with hugely controversial reputations. Humans are already 'playing God' in every facet of society. We can only really debate what the consequences of what we do will be. We can, and do, already make almost as many diseases as we attempt to cure.
You present many complex ideas in this question. I would also suggest that you might want to be prepared for a wide array of answers to what you pose. I think it should be noted that there might be a danger to use Shelley's work as a sort of gospel because I think she is well too aware of these dangers. With her father being a stoic believer in the power of the Enlightenment and her husband a zealous believer in the power of the Romantic era, Shelley writes a novel that critiques both in her depiction of modern tragedy. In her work, the Romantic thinkers who seek solitude and isolation might not be right, as this is what Victor does in the abandonment of his creation. At the same time, Victor having complete and unmitigated faith in Science is what causes the construction of the monster, and the destruction that follows. Both paths were proven to be realities that displayed each step towards creation was a step towards destruction. This modern phase of tragedy is inescapable. The question of whether humans should play the role of God will be highly debated on several levels. On one level, the question presumes God exists, which some might funadmentally question in its own right. At the same time, others would point to the fact that the progression of science has inherent benefits that cannot be fully recognized by a social order. The fact that we are here on enotes communicating was something not foreseen twenty five years ago. Such a vision would have been derided at the time. Hence, one is not in the totalizing position to suggest that science should be limited, for its wide ranging nature can transform reality into realms which cannot be foreseen. I am slightly confused by the first half of the question. I am not sure a viable case can be made to actually stop scientific progress in finding cures for diseases that are killing people. As long as people want to find cures, science will aid in that process. If we applied that logic to situations such as influenza and polio, millions of lives would have been lost. I believe that the question is more along the lines to discussing the limitations of science and the notion of "playing God," as opposed to suggesting that science should stop at finding cures for ailments and diseases.