OK, I know my 40+ years of being a machinist, combined with seeing aluminum radiators, car wheels and other aluminum parts corrode (galvanic corrosion) needs further education. My feeble mind was thinking that galvanic corrosion typically attacks aluminum in environments prone to having chlorine in the atmosphere (pools) is why austenitic stainless steel is the material of choice in marine environments. There are speakers which successfully use aluminum woofer cones and tweeter domes for exterior use, though those always have a specific anodized finish to inhibit that galvanic corrosion. It is because speaker cones and domes are so thin and galvanic corrosion occurs only on the surface that treatment (plating, anodizing) is paramount to life of the driver.
Yes, UV rays can break down certain polymers. Fortunately we have thousands of different polymers to choose from. Fewer are rigid enough for speaker cones, but there's still plenty to choose from.