I think it's worth pointing out that Spotify is a Swedish company, where iOS has a 58% market share, according to the site you linked. It could be that Spotify's main markets are ones where iOS has a more significant share.
"In theory, if the 30% cut was too high, developers wouldn't want to develop on the iOS platform"
The problem is not just that Apple is taking a 30% cut; it's that it's also running a competing service. Spotify are arguing that Apple are unfairly using their supposed dominance in one market (phones) to achieve dominance in another market (streaming music).