The maths may be correct, and if they aren't they can be corrected. But AFAICS you missed the point: those who are whining for 600 kg now will never stop whining, and will never want to give anything up. Not Vne (which in most countries is not regulated, today, for microlights) and not the liberties of self-maintenance and indeed nothing at all. They will go on whining and whining like little boys who are never content with the toys they were given. There is no use in seeking a compromise with them. Well, with some of them, that is. German microlighters in especial. They have already bended the rules to the very last inch, installing autopilots and glass panels and multiple GPS, which were far from the ideas of the original microlight concept.
They made these little machines into perfect travellers (low-cost, fast, reliable, well equipped) and now they realise they are at the limits so they clamour for a weight increase, so they can take them even further. That's all there is to it. If they are granted 600 kgs they will immediately start to clamour for 750 kg and ad infinitum.