If you have front-end development, the much better move in my opinion would be to master modern back-end development in ruby / elixir / go / etc... and become a true full stack developer. I made the move to iOS 5 years ago and regret it for a few reasons.
1. I became "stuck" doing iOS development. I recently had to switch jobs, and even though I have some server and web experience, it was more scattered and dated than my iOS experience. I was looking for senior software engineer positions and with my track record of iOS development, the only positions that gave me an interview were the iOS ones of which there were a lot less. If I had a history of server work, I have a feeling I would have gotten a lot more interviews even for jobs that didn't use the specific stack my experience was in. At this point, if I want to get back into full stack dev, I would either have to convince my current job to switch teams, or take a cut in seniority and pay.
2. iOS development is behind on modern UI dev concepts. Developing for iOS feels like going back to jQuery or Backbone.js from React. Things like a declarative user interface, automatic DOM re-rendering and UI Bindings are in their infant state in iOS, while they have been core concepts of web frameworks for a decade. As a result, a lot of iOS code out there is very messy. While the web moved towards light user interface components and centralizing business logic in the Redux state, iOS stayed with the MVC pattern which in practice often results in giant, untested view controller files very reminiscent of the big jQuery script files of the early 2000s. You can try to fight this by adding your own architecture and external libraries, but such approaches will always be second class citizens, not the default.
(SwiftUI is going to address some of these issues. However, its a newer technology and most existing iOS apps will be slow to adapt to it)
3. It will be harder to pass a FAANG interview. One of the crucial interviews is the "system design" interview which is almost always server related. As opposed to the algorithm questions which are easy to study for from a book, the only good way to answer these system design questions well is by having experience in designing and building such systems. If you stay on the client side doing iOS or front end, you won't be able to answer such a question effectively.
The reason I moved to iOS from front end was because I wanted to get outside of the browser to a more full feature platform. In retrospect though, the obvious platform to transition to is not iOS or mobile dev, its the server. The server is a full feature platform which gives you the ultimate freedom to do whatever you want. And while frameworks like Ruby on Rails or React come and go, I found that job openings were pretty indifferent between the specific framework you worked on as long as you have server experience in general.