I'm honestly surprised at the level of after purchase service expected.
There are many, many factors that go into how fast and why gold plating wears off. The warranty is 18 months, which, personally, I think is a fair amount of time to determine whether or not an item is defective, versus showing normal wear.
I do think that Marc Jacobs needs a more visible repairs department. I personally find the official website a little difficult to navigate, and if I didn't frequent this forum, I wouldn't know the policy.
I will say that my experience with repairs has been wonderful. I would never expect to be able to send in a ten year old bag to be repaired, but they have always tracked down (even if that means having someone at the warehouse in Italy look) what I need, if they have it, so that I can fix something myself.
In fairness to MJ, in 2008 they did look into a lifetime warranty for a fee. Obviously, that was never implemented, for whatever reason. I assume there wasn't a way to make it cost effective.
Being a modestly priced designer (by premiere standards) with a limited warranty apparently hasn't worked out well for the company, though. Maybe it was just previously too "in the middle" - not small enough, like Mulberry, or large enough, like Hermes, to provide a generous repair policy, and not cheap enough for the majority to be comfortable purchasing a bag for that amount without one.