- Octave won't die. It won't die even if you ditch it altogether. A lot of companies depend on it because licenses are free and that makes it easy to do massive parallel computations by launching thousands of instances. Matlab doesn't let you do that. As long as it stays GPL, the employees within those companies and academic institutions will continue to contribute to it. (Python is competition, but there are still plenty of Octave supporters.)
- You might consider working for a company that is built around Octave. They would likely gladly welcome you as well as support you in spending a percentage of your time to maintain it, since their business depends on it.
- Build something that people will pay for. Like a decent GUI for Octave that does everything Matlab's GUI does. Or a few awesome closed-source toolkits. That alone might support your personal income needs.