Therein lies the problem, that makes it a masterpiece and an innovative piece of film at a certain point of time. If the things that made the movie great are new techniques and not the quality of the film itself. A good film is a good film period and for all time, if you have to qualify it with "for its time," then its not a great film. Years from now people may watch the first Matrix and wonder what the big

ing deal was about it. If once you strip away all the innovative use of effects that were copied and proliferated, if the core film itself is not interesting, then it was never a good film.
First, Citizen Kane is a great film in and of itself. However, the innovations of Welles - his gifts to all future films - are part of what make it arguably the best/most important film of all time.
But even subtracting that, great films can be great for different reasons. I think Speed Racer was a visual masterpiece, to the point that I don't even care about the plot at all.
Or, take something like Birth of a Nation. It's a classic film, powerful, important. But it's plot is now so antiquated that most people wouldn't want to watch it (The KKK are essentially the heroes of the film). Likewise, Leni Riefenstahl's WWII Germany propaganda films are still textbook examples of the art.
All art ages. Modern media, especially, caters to specific, modern demographics. Everything from opening to credits, to hairstyles, to plot mechanics, stereotypes, jokes, pacing, camera technique, and picture quality vary greatly even across 10 years time. But that doesn't mean they aren't great anymore, nor does it mean "it was never a good film".
Film is a language that evolves and changes with the times. For example, The Usual Suspects brought twist endings back into vogue, and then a few years later the Sixth Sense brought it mainstream, and then for years practically every film had a twist ending. Likewise, Pulp Fiction turned traditional narrative upside down and inside out, which lead to dozens of mimics as well as influencing other narrative ideas like Memento.
The thing is, if you didn't see the 'original', you'll never recognize when lesser films are just copying another director's (or writers, or cinematographers, etc) innovations. While that may not be important to you, it's important to many enthusiasts.
Likewise, just because the interfaces of all crpgs from the 80's are now archaic and near unplayable doesn't mean they weren't good/great, both 'for their time' and 'now'. It just means you need to work at it a bit more.
Citizen Kane is great. I haven't watched the Ebert commentary yet but I'm pretty sure it's on the DVD version that I own.
It's worth it.