There are many closed source applications that have been removed from the Google Play catalog for various reasons.
Often this reason is the cessation of further development and maintenance of the application.
Why don’t these people make the code open to everyone if they don’t need it anymore?
This could bring great benefits to all people, but everything that was once created simply disappears.
Maybe you could find some answers if you have a look into apps that actually did what you suggest and see how they did through time. Comes to my mind, for example, Astrid, a to-do app that became FOSS after a good run in the early android days.
It’s called Tasks now (in case anyone is looking for it).