Everybody always talks about how "cheap" and "flimsy" the factory followers are, but I've never seen one break or crack. I've never even come across a
picture of a broken factory follower. One of the Boy Scout camps my brother has been involved with has a clay range and a pile of the most beaten-to-crap 870s you could imagine - all with intact factory polymer followers. Remington puts these same parts in their Police and military 870s, and there's no way they would be doing so if these followers were really as fragile as all the armchair gunsmiths on the internet claim.
Sure, anything made by the hands of man can break, and a higher-power spring will increase wear, but that goes for aftermarket followers, too. And some of these "upgraded" metal followers can actually make things worse. Heavily-used aluminum followers can become quite deformed from repeatedly knocking against the steel follower stop at the mouth of the magazine, which does nothing to improve function or reliability.

- Peened Brownell's Follower_SJ.JPG (16.36 KiB) Viewed 4918 times
Aftermarket steel followers are harder, but also much heavier, and they can actually end up damaging the gun itself over time.
The Remington factory polymer followers will never hurt the steel components of the gun, and at just a hair over a tenth of an ounce, their extremely low mass greatly reduces the amount of inertia involved in any recoil-induced movement or feeding, so impacts are actually less severe. And since they're just $4.12 each, you can buy several spares for less than a single high-priced aftermarket follower would cost.