E-ink is not necessary for a long battery life - the Amazfit Bip, has a long one (up to 45 days).

An Amazfit Bip equivalent, with an open system would be a great product at least for a niche (Amazfit's interface and functionality is arguably poor).

edit: has this post been downvoted with information at hand? The statement above is technically correct: the Bip has an LCD (see specs), and it lasts very long; therefore, E-ink is not the only option for a display of a long-lasting device.