The news of Mailbox’s untimely euthanization, together with your observation that there is no single client that spans both iOS and OSX, comes a few days after Mozilla announced that Thunderbird might ought to have a future. I wonder if the path to a great 2016 OSX client runs through Thunderbird.
I still use Thunderbird for Gmail on my laptop, mostly because I like local archive storage and searchability, and I do enough message-level encryption and decryption, especially when traveling, that Thunderbird plug-ins are a godsend. And it’s so flexible and speedy. If Thunderbird caught up to the current mobile feature and function landscape, supporting newer capabilities like Inbox’s snoozing, it’d be an optimal client for me.