This would be a very handy script, but if nobody is able to help you here, I’d recommend looking into MeshLab. It has command-line ability (server) and will provide you with much better results than Blender’s decimate modifier.
Let me plug my paid add-on here (since particl3s said he is ok with a paid option) for future needs with complex operations. Io-Guru can import and export objs enmasse.
You can apply a modifier to one of the imports then Ctrl-L to copy the modifiers to the rest of the objects, then export all the objects in the scene with a single click with the add-on.
And thanks to abdoubouam’s solution, take a note of his script in the .blend file (in the script editor). It can be very useful if all you need is basic folder based processing.