Fluorescent Tube Cathode Shields Question

According to Lamptech, the purpose of cathode shields in fluorescent tubes is to decrease the effect of mains frequency flickering around the electrodes and also help shield the glass from sputtering for better lumen maintenance. In the picture on Lamptech (and also in pictures of any shielded germicidal tube I have ever seen) the shield is electrically isolated from the cathode. This is done by attaching it to an isolated support wire coming from the pinch seal. I have a question:

Wouldn't it be better to connect the shield to one of the cathode wires? This would make it act like an anode plate (like in the NA-1 sodium spectral lamp) and probably increase the lifespan of the cathodes. Why don't they do this? Also wouldn't the isolated shield build up a negative charge (like a floating CRT deflection plate) and then repel the flowing electrons near it (increasing electrode voltage drop)?

Also: What are they made of? Just out of curiosity

Thanks!