The Delrin Simplex derailleurs aren't nearly as fragile as you would think. I rode a PX10 stock with them as my commute bike for a couple years. This was about 16 miles/day, plus I'd take it for occasional weekend 50 mile type rides. They were still working fine when i gifted the bike away. I'd be inclined to just ride it for a while and see how it goes. Granted this was ~ 20 years ago, and that plastic is twice as old now.

Obviously if you abuse them by shifting 3 cogs at once while under pressure up hill etc, they will break eventually. If you're the type of person that regularly breaks stuff, perhaps you should swap them out now. OTOH, if you're not a basher, give 'em a try.

It's really the front ones that are more inclined to give up the ghost. They work fine till the front mounting bracket piece fractures.