There is a lot of nonsense written about lenses not being compatible with the 40mp sensor. In any system the resolution is determined by the sum of all the components in the imaging chain. Of course, a higher resolution lens will perform better, but an18-55 combined with the 40mp sensor will give better results than it did with the 26mp sensor. I have one of those lenses and a Fuji X-T5. It will outperform your X-S1 with ease.