It is not that simple. Android apps are written in Java against a completely different API. To port such an app to QT means a complete rewrite of the app. From what I understand, the idea of Anbox is to simply provide this APIs and the dalvik jvm in a chroot/container. All free software, it just allows you to run apps written for the Android userland on a system with a regular GNU userland.
This has all absolutely nothing to do with the problem of proprietary software. In fact there where quite a few proprietary apps in the Ubuntu store which where native apps running in the GNU userland.
The important thing here is that the OS is all free software and that the choice to give up your freedom by using proprietary software is one the user can make because he has this freedom which comes with the full control over his device in the first place. This is true with and without Anbox.
Now I recognise that the possibility to run Android apps will increase the probability that users run proprietary software on UT simply because it makes them available to the user. But they would run it anyway if they really need it but probably on a OS which does not respect the freedom of the user and may spy on you. So if they run it on UT instead of a stock Android full of proprietary spyware they at least gain control over the device they own.
For the people who want to stick to free software only, Anbox opens access to a lot more free software than they would have on UT with native apps only. This could make a big difference to make UT a viable choice as a device for them.
I also don't think this will in any way discourage people from writing native apps. The true power of UT is that it has a full GNU userland with all the tools people are used to from the desktop. But from what I see there has to be a lot of work to be done to make developing native apps viable for mere mortals again.
Sorry for the wall of text