You can keep using PyMC3 with confidence, we are still adding lots of new features to it.
As for long-term development, we are currently looking into different new backends, with backward compatibility as one of our priority. We hope that for pure PyMC3 code (i.e., does not contain theano ops) it will work also under PyMC4.
As for the PyMC3 with theano as backend, we are aiming to have a stable long-term release (personal projection is in ~2 years), with continuous maintenance of theano either on a fork or on the master project.
More information will follow, but we will surely be around for a while.