I almost switched away from ProtonMail because of the original interface. I stayed for the security and privacy benefits. The new web client is amazing! iOS and iPadOS have yet to catch up, though.

The current iOS client for ProtonMail is slightly confusing and foreign. The layout is fine, but the small button size and the low-contrast colors make it unpleasant to use. From a development perspective, a custom UI doesn't seem easy to maintain - I think this is proven by the fact that the Android app was updated just 15 hours ago while the iOS app hasn't been updated for over a month.

Swift is a very easy language to learn - I'm starting to pick it up as a first language quite quickly - but it is a fast and powerful one too. SwiftUI allows developers to create frontends in seconds with little effort. Documentation on Apple's developer site is great and Apple provides amazing tools development tools like Xcode and the iOS/iPadOS simulators for free.

I think the current app should be completely reimagined and reimplemented using SwiftUI. Though I am no developer (yet), video sessions from WWDCs past show how easy it is to develop apps that integrate (visually and functionally) with the platforms while still being unique and representing the brand.

Furthermore, the current app doesn't align with the new ProtonMail design system (found at design-system.protontech.ch). The first page says to "avoid reinventing the wheel for Front-End," and to "[improve]/[simplify] Front-End maintenability/scalability." Rewriting the Proton client for iOS in SwiftUI would do both. You wouldn't be reinventing the wheel by using an existing UI kit for the app's interface. The app would be easy to maintain - if I'm not mistaken, SwiftUI apps are easy to write and understand and automatically reflects the latest UI changes when rebuilt for the new platforms. It would be easy to scale - SwiftUI apps automatically adapt to fit iPhone and iPad screen sizes and Mac Catalyst can automatically turn the iPad app into a Mac app with the click of a button. Most importantly it would improve the user experience with ProtonMail, making users more likely to stick with the service.

I would suggest rewriting the UI from scratch using the latest interface guidelines and recommendations from Apple. I think a rewrite in SwiftUI would benefit both Proton and the user base.