It would be fine by me if they switched to cardboard packaging. I take such good care of my stuff that I actually still have gameboy and NES cardboard packagings from those old games... so any game collector who is really protective of his games could certainly do the same.
But I think a good compromise would be to do like the old Gameboy games used to be, where they were in the cardboard box with the manual, but the game itself would have a tiny plastic case which the cartridge would fit into. No one can seriously say those tiny little gameboy cases used a lot of plastic, so I think that would be a great compromise here. DS games are quite a bit smaller than GB carts, so they could get by with even less plastic for their cases.
If you ask me, the DS plastic cases now are ridiculously huge and bloated and that's very wasteful with plastic. The carts are small, so the cases should also be small. I know they need to have big cases in order to get attention from shoppers, but there's no reason that can't be done with cardboard. I think that would be a great compromise to do things this way.
As for Wii and other Disc based games, I can see the need for big cases, because discs are huge, but why does the material necessarily have to be plastic? I'm not sure what options are out there, but we have to remember plastic and cardboard aren't the only materials that exist. Like, what about cloth pouches or something like that? What about, uh... wood or something?