I don't think modding is ever gonna go away. Even in the future when pc's are intergrated into just about everything we do someone out there will want to mod it.
And for buying pre modded cases; I don't think it's a bad thing at all. Some people want something cool, and they want it right now. So they go and buy it, they enjoy it, and people ask "where the heck did ya get that"., because they have never seen it before. Most of the average joes out there don't mod, so when they see something like that it impresses them. Granted the owner doesn't get the full satisfaction of saying "buy that, heck no man I made that".
I own a pre modded case, but there isn't a single gadget in there I didn't mod. All it has is pre installed windows with the cool looking brass screws holding them in. I just liked the way the screws looked. I modded my cdrw to match it even. But for the windows, I etched every last one of them Ziggey style with UV paint and the works.
I was totally clueless to modding before I read an article in MaximumPC which mentioned overclockers.com. Shortly after I visited this web site, every warrenty to my computer parts were void. I just couldn't leave anything the way it was.
I'll stop babbling for now. These are just opinions, and a personal experiance.