We share your concern, and we’re keeping a close eye on the alternatives. The most promising one is JMAP (JSON Meta Application Protocol), an open standard developed by an IETF working group. It’s a modern HTTP-based protocol with JSON payloads, and it doesn’t depend on any single vendor.
The core protocol and email sync have been published as IETF standards, and we’re starting to see them implemented by various email systems. The standard for syncing contacts has also been completed, and standards for syncing calendars and tasks are being worked on. There’s a published framework for sharing, with an extension for shared mail folders in progress, and a published extension that lets JMAP run over WebSockets for real-time push updates.
While we aren't quite ready to implement JMAP in SmarterMail, we are keeping an eye on the working group's progress. I'm optimistic that JMAP will be a good fit for taking the place of EWS as Microsoft leaves it behind.