The answer to all of the above is: 'Don't upgrade'. As it stands ST is an excellent editor with a rich plugin system around it, even if the dev decides to call it a day it's unlikely it's going to stop working in the next 2-5 years, and additional features can be introduced via the plugin system.

I do understand the concern though, but for a lot of people (myself included) ST is a better solution than a non-proprietary solution.