Do the older derailleurs even have a constant cable-throw throughout their entire range of motion? It wouldn't take much deviation in this to cause even the best indexed shifting lever matched to the derailleur at one end to not work once it got to the other side of the freewheel/cassette. Why would the manufacturers put so much effort (cost) into pre-indexing derailleurs to get them to be perfectly linear across the entire range of motion in a friction system? It would be a lot of effort to do this when it wouldn't really add anything to friction-shift system. A tiny amount of variation couldn't be felt by the rider in a friction-system as it would be hardly measuraable. But in an indexed system it would make ALL THE DIFFERENCE in the world between a system that works and one that doesn't.