I think step one is that these specific clients should stop comparing their websites to the Googles and Amazons of this world. How is that a valid comparison, right? I think Amazon analyzes every slider over and over. Did you notice some of the sliders slide on demand only (so are basically a hero image / static image) and others slide automatically, showing a tad bit of the next slide already, always with the option to slide back? And in most cases, there is a clear, static call-to-action on that page, next to the slider. That could, for instance, be a list of recommended products. No product page has a slider like that, as it is focused on conversion.
There is always an exception to the rule, but you can bet on Amazon testing that exception all the time.