Hangouts on Air are not exactly disappearing. They are no longer to be a part of Google+, where they began a few years ago. The identical functionality is available on Youtube Live streaming. Live Events that look exactly the same as the old HoA can be programmed with the choice of using the “Quick” option for HoA or by using an encoder with the streaming settings you prefer.
This said, building something on someone else’s platform will always be subject to a rug-pulling exercise, as we saw with Twitter developers.