galvanized metal roofing, durability?

[Durian Fiend]

...Everything wears away eventually, including acrylic coatings. I hope the birds will not poop on the roof, or claw away at it.

They might occasionally poop on it. Same with rats, although I've never noticed any type of poop on my roof when inspecting it.  Of course it would usually be washed away in short order with the abundant Puna rainfall. 

There's no reason for any critter to scratch/claw at it. 

The most common spot for metal roofing pieces to rust is at the drip edge. A new metal roof could very well last your lifetime, unless perhaps you're mid 30's or younger.