I think you need a certain density to make last-mile-bike-delivery to work. If it's 2000 deliveries per square mile (i.e. very dense), then the current model of human-in-van is more efficient. If it's sparse, say 5 deliveries in 50 square miles, then the human-in-van is also the more efficient model. In between there is a sweet spot where the van + bicyclists becomes more efficient.

So you need to find that density sweet spot and the squad of bicyclists to make it work.

I don't think it's feasible.