Bare aluminum is okay as a fender material--the corrosion is self-limiting, to a point. More to the point though... It won't be corrosion that kills the fender. The drawback of metal fenders is them getting dented/dinged in a way that they can't be repaired from...plastic fenders are much more forgiving of that. Aluminum fenders, in particular, are difficult to pull dents from, since they would rather buckle/tear.

Long term metal fenders are typically steel (stainless, now, though Wald still makes chrome-plated ones) or brass, since they tolerate such repairs better.

At the end of the day, though, all fenders are consumable items. One day, they will crack around a mounting point, or buckle as debris is jammed beneath them, and you'll be shopping for new ones. It may take 10 years, but it will happen.