Most modern deraillers are not meant to last long. Just long enough to get you out of the warranty period - or until the "improved" version is marketed.
I repair old deraillers all the time. Huret used to use BALL BEARINGS in their jockey pulleys. They could be cleaned, greased, and reused. Whenever I come across a butchered Huret, I snag it just for the pulleys. Now that I think about it, I can't rememeber the last time I came across an old steel Huret derailler that was broke beyond repair.