Simplification and selective detail has to look coherent and cohesive. What you have now makes the difference between the foreground and background too jarring.
There are ways to make something “feel” finished without rendering it to death. You have to think about the basic shapes, the smaller shapes, faking the feel of polish with textures, and doing some accents in the smaller forms that offset the larger forms.
What you currently have in the background is basically all medium level forms, no textures, and all painted with the same/similar sized brush. Also, the two elements (foreground and background) are so close together in distance that it makes no sense visually that the background would suddenly be so unfinished. If it was a background far away, the effect wouldn’t be as jarring.