Bottle dynamos wear down -- their other name are "friction dynamos". That being said, your one does seem excessive and may be a result of your friction wheel rubbing against the high end of the tire, near the treads. This area tends to be dirtier as well as has tire treads, so this might be causing premature wear.
Since you ruled out a hub dynamo, I'd suggest instead that you try another cheap dynamo but try to make sure the friction wheel hits the tire sidewall and not the tread. Notice how the sidewall has a friction track ||||||||||| that's intended for the bottle dynamo.
To the greatest extent possible, the dynamo body and friction wheel should be parallel to the bicycle wheel when engaged. The better friction dynamos use parallelogram linkages to accomplish this, the cheaper ones a simple hinge which will mean the two wheels are always at an angle to each other, causing a hot spot and bearing wear.
As far as steel vs plastic friction wheels. It's expensive to procure a plastic with the right friction coefficient that is hard wearing, won't melt, and won't damage the tire. Thus, cheaper dynamos go with a cheap metal casting. As @ojs notes, you may be able to get replacement rollers for higher end units.
If that fails, I might also suggest rechargeable battery lights. You can't go through many 10~40 eurodollar dynamos before a single 40 eurodollar USB rechargeable light makes more sense.