It’s very rare that I get to have this conversation with a client.
“Launch it already. We can always improve the app the next week and the week after.”
Software is a continuous process. It needs constant improvements. There is user feedback to be worked on, bugs which are yet to encountered and design which needs tweaking.
If you have 5 out of the 10 features ready(well tested of course), you should launch that app / product you are working on. You can always improve it.