Inhumans are a substitute for mutants in Agents of SHIELD. But they were criticized in season one for Scorch because the audience didn't want random made up characters who got powers through freak accidents. So the Inhumans fill that role. But they can't use X-characters there so I don't know what's expected. However, the Inhumans show is not using the Inhumans as substitutes for X-Men at all. In fact, the best way to ensure they're not a substitute is to hope things like Inhumans on TV, Royals in the comics, etc. are successful.
However, it's noteworthy that they were not retconned into being Inhumans at all, despite what some people thought would happen.While Fox and Disney worked out a deal for Scarlet Witch and Quicksilver to appear in the MCU, they were retconned from being mutants because Disney doesn't want to even reference a term that isn't their Intellectual Property.