I just don't get it. I do understand how it's a better type pump in that it doesn't mix up the influent as much as another type would. No doubt an Archimedes screw pump. Cast iron is the standard material of cosntruction for sewage and wastewater. Virtually all of your sewage and wastewater pumps will be cast iron with a good two-part epoxy coating.
Plastic is nice on paper, but a screw pump that size is impracticle.
In addition, plastic pumps can get quite expensive. Can't comment too well on how well the pumps are holding up, but the bearings wearing out is probably more of an issue with the bearing design itself as opposed to the materials of cosntruction of the pump. Aluminium is rarely used as a pump material for such applications. What perhaps could be considered is stainless steel or wear resistant cast iron like NIHard. Normally the castings are manufactured as an integral piece and not in segments as mentioned by you.