It really is not a valid argument at all. The life expectancy has increased over the years. Evolutionarily it is not very useful to maintain anyone beyond the reproductive age where they can maintain the species. But we have gone beyond just survival of the fittest. Covid does also kill younger people and also younger people get very ill with it and there is also long covid. There is also the possibility that a virus that keeps mutating can become more deadly and resistant to vaccines. Simplifying the problem to mean only prolonged survival for older people is a serious oversimplification.