I like your point. It's fascinating.
Right now, I need to look into the implications of running DarwinMail as a browser extension would be. To be sure your security would be maintained, the core functionality is not lost and DarwinMail remains usable (landing page with about, features, testimonials, pricing etc).
Thank you for your suggestion!
I'd not say - go and build this as an extension on mobile; but if you have the code built to run as an extension, it should be fairly quick to repackage it as a standalone mobile app, i.e. get a dedicated instance of the browser that runs your app).
Another successful example of this model is LastPass - they deliver the functionality via browser extensions and mobile apps, but they handle all the payment and marketing and CRM via their own product site.
Thank you for the feedback!