I believe the failure mode is that the impellers shear off the spindle, and there is a theory going about that the stress of detensioning and retensioning with a new belt is actually the prime culprit in causing this. Thats why you should get a new pump with a new belt.
....That's what I was independently told and later confirmed by my friendly VW dealer.
Not sure about that.
I think the prime culprit is the hot and cold incessant cycle the plastic has to endure, which makes the plastic age harden and brittle. The Bearings would take the brunt of the De tensioning/Tensioning.
The impeller is on the shaft and unless the shaft has the capability to force the impeller to be stressed against the orifice it sits in, the the only forces the impeller is subject to is the rotational shear force the coolant can induce, and the afore mentioned H/C cycling.
Regarding the H/C cycle; there is some truth in what you say there, it contributes to the failure but is greatly accelerated by the new belt. This isn't unique to VAG, its a common problem with most water pumps / engines. The most common time for pump failure is shortly after the belt is changed.
To be pedantic its the impeller that induces a force on the fluid, the fluid merely provides resistance.
I suspect that the tensioing / retensioning causes the shaft and impeller to move slightly off centre which adversely changes the flow around the impeller, concentrating stresses where they are not designed to be and ultimately resulting in failure.
So the impeller is rotating as you as fit the belt? It doesn't matter where the impeller sits within the void within the engine block, unless it touches the block, nothing will affect it. Unless the seal is gapped, then you will get a leak, the pump will still be rotating. The impeller will still be doing its job.
I have changed 2 VW water pumps recently, and both broke up and crumbled in my hands, the plastic has become very brittle due to H/C cycles, nothing to do with differing flows around it.
you are right, the fluid will induce a resistance to rotation, this is a shear force trying to twist the impeller off, but I think we are talking about the same thing. Tomatoes and red things!
in the pic below you can see the cracks and how aged, brittle and crumbly the plastic has become.