simply say Malthus is irrelevant when his assumptions (positive relationship between wealth and birth rates, population grows faster than economy) don't hold?

This is the correct view; his argument is practically deductive, and the only way around it is to take one of his escape holes: people collectively choosing a higher standard of living rather than offspring. The real questions we should be discussing are:

1. why does the demographic transition exist?
2. will it last indefinitely?
3. if it willn't, when does it end? And will it reverse itself to high-fertility and either a population increase or a decrease in standard of living?
4. What repercussions does the end of the transition signal?

Question 4 leads us right into Robin Hanson's crack of a future dawn and Dream Time scenario.