well Magpul is aftermarket and plastic, since they use a special polymer , and it's tough as nails.
And yes they are in fact cheap in price, not cheap in build quality.
Furthermore it has a really good follower that doesn't tilt like the one that came from the factory with my AR..
Those magpuls are easier to load, the follower doesn't tilt, and they are durable.
As for wearing out the springs.. It's all good if one shoots often and rotates the ones one shoots..