A natural use case is to have open one document per window. TeXStudio seems to support only tabs as an interface to multiple documents. This ticket requests that enhancement, that tabs be removable to their own windows.
Thanks for your proposal. TXS is stronly build as a single-window application and that's not easy to change. You can start another instance using the --start-always command line option. However, these are separate applications which concur for the settings, e.g. each instance will write it's settings and last session info on closing. If you're ok with that, maybe this is a solution for you.
I agree that your proposal makes sense. But I don't think we've got capacity to make any larger changes towards that. We have a long list of improvements and we need to priorize since time is limited.