So for your information, I'm running a new WIA motor, I've never replaced my switch nor done any wiring work for the wipers, and I'm still using the stock mechanical washer pump. I replaced my wiper arms with new arms because my old wiper arms had springs that were weak which lead to streaking. (I would be concerned about that if you went to weaker springs) And I'm running 18" Bosch Clear advantage beam blade wipers. I will admit that if say I turn the wiper switch on by accident on a dray windshield they'll run slow, but 18" beams are going to have a lot more resistance dry than conventional 16".
My point to all this if your wipers are slowing and stopping there is a problem somewhere, I've gone over the most common stuff in my earlier posts in this thread. You don't say what you're running for wiper blades, so that could be a contributing factor, but I think even if you do something like install lighter springs, it'll catch up to you because whatever is causing the problem will likely get worse.
I will also add that I have seen wiper linkage that appeared to more freely, but would bind up when resistance was put to it. It is possible to take the linkage apart and lubricate everything, which will reduce resistance. It's 40 years old and likely never serviced, you know it's lost at least some of it's lubrication.
Well whatever path you choose to go down I hope you get them working properly again, and report to us what you try.