Sigh…
Any developer will start working on new projects when a major project is released because some of their staff is now free to work on new things.
But…
Assuming they will abandon their previous work is folly. We have had the sim for two years now. Stock planes receive updates. Maybe not as quickly as payware planes from some developers, but often enough to make it clear, even Carenado (not known for quick or consistent patching) will continue to patch and fix planes when contracted by MS/Asobo for exclusivity. Their contracts clearly involve keeping the plane in working order, updating it to be compatible with future updates, and quelling bugs.