More to the point, I don't think it's graphics quality that determins a game's lastability but rather the amount and quality of actual content
It is more that Graphics is what "Dates" a game and seperates the difference between an "Old game" and a "New Game". Once that factors out we could start seeing a shift in game focus away from technical graphics as the community matures.
Also while there will only be improved graphics eventually the improvements will be meaningless (probably around the time Crysis or Heavy Rain graphics become standard... after that... Graphics don't make significant improvement).
"If graphics had any influence on game lastability, surely modern games would last many times longer than old classics, yet the exact opposite is true, at least in my experience"
Your kinda thinking... hmm... a bit off. Modern games put more on a focus on graphics then many of the classics seem to be today. When people play Classics they ignore technical graphics and when people buy modern games they are seeking the "Next evolution in gaming". So it is more about how the market treats them both then Graphics having a moon like pull on the tide that is our wallets.
There are MANY lost gems that never became classics as well though.
My idea is that once games cannot seperate themselves from technical supperiority then they will have to shift their focus in gameplay (Well once the community realises that "Hey, even though this game has twice the graphics it doesn't have twice the worth). which will make the production of lasting classics more frequent.