I took Uber 5 times and here are what the drivers told me:
Those might be Uber's secret army giving Uber their flexibility.
You nailed it in your post.
Notice that none of the five drivers mentioned they got a lease through Uber Xchange, which implies none of them are relying on Uber to make their $600 to $800 monthly payment which gets deducted in weekly increments from their Uber checks.
The key phrases from your post:
Driver 1 and 2: "...retired software engineers...only picked up local fares..."
Driver 3: "...young engineer...used Uber to get pocket money..."
Driver 4: "...used Uber temporarily...did not interfere with job search at all."
Driver 5: "...drove for fun...only when he had time..."
Obviously, these five drivers aren't candidates for the Uber Xchange. All five were NOT desperate for money or trying to rely on Uber just to survive, unlike the candidates from Uber Xchange. If they were, then you'd probably have very different responses.