> I'm increasingly seeing increasing number of random personal vehicles drop off my retail packages that were shipped via UPS/FedEx to a central hub

I have also seen this, but I’m pretty sure these people are essentially employed as delivery personnel, and their cars are acting as small delivery trucks. I’m not sure how the cost for this work out. Maybe FedEx/whoever is closing the gap when they can’t get everything onto real delivery trucks and this is more cost effective than buying more trucks and hiring more drivers?

I think last mile is an interesting problem but OP seems to be intending to build full transit logistics infrastructure built on casual labor, which seems unlikely to pan out.