Nothing last forever, but i have never experienced or heard of anyone who had have this problem yet.
How do they know its been erased? It might be other things, like dirt, that makes the game not work.

Regular games will be fine for probably long past your life time.  I am sure like Michael said you probably read something about protos.  The should start slowly erasing themselves after about 20-25 years.  The information will slowly decay away, and you are then left with a basically worthless piece of plactic unless you have the proto dumped so you can make a new chips for it.  This is why I don't collect protos, to much hassle.  I have 4, and think I may sale or trade those off for something soon.

I read that NWC uses EPROMs also.  Can anybody verify this?

Yuppers, and lots of test carts, and actually lots of unlicensed games (like a lot of the color dreams).

I think this is a good idea and the only way to preserve protos. Everybody should dump the ROMs and save them on a CD or whatever. After the original chip lost his data, a new on can be produced and the proto should contain most of its value when the original chip is sold together with the repaired game.

Well, they're all dumpable, games like airball are just unusable without that special hardware (no emulation is possible without emulating the special hardware and it's impossible to make a repro without reproducing the hardware).  It's still worth dumping for a backup to redo the eproms.  It is probably easier on a game like airball to remove the eproms and dump them directly.

As for "against the rules":  Yes, dump it.  If it's never been dumped, dump it privately and don't spread it around if you're afraid of the value going down.  You can usually refresh eproms and get more life out of them (erase and rewrite them, but you usually have to unsolder the chips if they're soldered in so it's a pain).  You usually need something like a copynes to dump the games unless you want to take the eproms off and use an eprom programmer (again often needs unsoldering for the eprom programmer part). 

If an eprom goes completely bad, replace it with another eprom (if you have a dump!).  As long as you get an eprom that doesn't raise any red flags, nobody will even know.

If anybody wants their game dumped privately, I'll be happy to do it if you pay the shipping both ways.  If the eproms are socketed, I'll be happy to refresh your eproms for free.

I don't know if you're talking about a Proto or a regular cart, Nintendo. If you're talking about a proto, yes it can loose it's memory just by sitting there. 

  If you're talking about a regular cart, there's a difference between being erased and stopped working.  Most carts that stop working still have perfectly good rom chips (where the game is stored).  If it's worth paying the shipping both ways, I'd be glad to take a look at your cart.