If IMAP access is not going to be possible due to Protonmail's local mailbox decryption (which is what I understand to be the case, though I could be wrong?) then in my opinion high quality mobile apps are absolutely vital if this service is going to truly take off.

Access needs to be easy everywhere and the size of the trade off between convenience and security needs to be minimized as much as possible.

Decent iOS and Android apps would mitigate the loss of IMAP and strike the perfect compromise between security and convenience for me.