Yeah, you'd probably have to start with a small user base from scratch here since I suspect most users are just waiting for the app to be released first before committing. You might have to take the risk and get the annual subscription, develop and release the iOS app and build the user base. It'll likely be slow at first, but word of mouth gets around and the user base will grow enough until its worthwhile and sustainable.
And if it doesn't work out after one year, you can cut your losses and not renew the subscription. But I suspect it'll probably start becoming popular before the end of the first year.