Just to be clear - it’s the API that’s shutting down, not the app. Not that Google has put in effort for the app either, it hasn’t updated since health connect afaik, but health connect is the health and fitness tooling going forward.
Fitbit has health connect support now, so even if they shift and drop Fit (I hope not, though I also hoped they wouldn’t kill the web interface), and make Fitbit the main Google fitness app, it will still work with Fitbit as the app.