iOS is my main mobile OS. I have tried GrapheneOS, but at this stage, iOS offers better compatibility yet does not compromise on stellar security. Perhaps I will switch one day.
As stated, iOS offers the best of both compatibility and security. Apps are frequently developed and maintained. Companies need to support it and thus need to confirm with strict Apple guidelines and iOS privacy and security.
iMessage probably keeps me on the platform the most, but is far preferable than SMS or other non-secure messaging.
iOS may bring features late, but usually does them right and/or prettier. For example, widgets and an app library. Or maybe I am biased.