The mass loss of quality is because there is a long standing bug in the decals placed. You can place ai. files. You can place 5000 px files. It doesn't matter. They all render low quality. Adobe has known about this for a very long time. They are promising fixes for the next version, but that has been said before.
Since the beginning, any image you try to apply doesn't render at full quality, it's a bug. It has been addressed by users countless times over the past year, but has yet to be fixed. You can apply the highest quality graphic and the problem remains. You'll also notice that on many models you'll see a white outline on parts of your 3D model, making it not possible to use for professional work, this white line ( not really a halo ) will appear on rendered models often. There is no way to remove it. This is also a bug. This white line appears on the exterior outline of the model in areas. We must remember that this problem is new, it will need some time for it to evolve. We must remember that other programs, like Cinema 4D have been around for what feels like forever, and is a much more advanced and honed application. Let's hope Adobe decided to keep working on Stager and Modeler.