I think most of them just go ahead and replace the Delrin Simplex's on their bikes with better/later model ones..... I guess what I'm saying is, when the old plastic RD on your bike asplodes or wears out in some way, it's really a very good excuse to upgrade to an all metal one anyway...cause next time it might not be just the jockey wheels cracking, but maybe a main pivot body or derailleur arm that could result in a unexpected sudden stop of your drivetrain and worse....
I think I can actually see some lightening/greying on the plastic of the main pivot bodies on your RD in the pics......that is not a good sign....