A work or a creator becomes Deader Than Disco when Hype Backlash makes it go from hugely popular to almost universally hated, with little-to-no chance of ever making a comeback. This is the most important criterion. However, there are many ways that this can happen.
Technology isn't the only gimmick available. A work can also attract audiences by showing controversial content or addressing hot-button issues, and if there's little else underneath, audiences in later years can wonder what the big deal is once that content is no longer considered edgy. Again, stories like The Catcher in the Rye, Night of the Living Dead (1968), All in the Family, Watchmen, and Doom, while known for being landmarks in terms of breaking taboos in their respective genres and mediums, are still watched, read, and played nowadays because they had other merits in addition to their content.
On an artistic level, a work that spawned a revolution in creativity in its medium could easily be outdone and consumed by the works that came in its wake. What was once a radical new take on an old plot device could become a Discredited Trope or a Dead Horse Trope in its own right. Later works may take the same story and do it better, causing the original to be forgotten. A good deconstruction of a genre's conventions can cause people to reevaluate their enjoyment of even the better works in that genre, causing them to go from acclaimed to reviled. Sometimes, a creator can do it to themselves with a sequel that turns out to be even better than the original was that totally overshadow the original's merits and make its flaws stand out more. Alternatively, copycats can simply run those conventions into the ground and cause people to get sick of them, to the point where even the acclaimed works that popularized them can see their reputations suffer. Or, people can tune into a Trope Maker to see what all the fuss is about after watching the copycats, only to end up disappointed when the work in question isn't all it's cracked up to be in their eyes. Or worse still, the original points out the genre's flaws before they even had a chance to be set in stone, causing people to view the entire genre with a more critical eye.
And on a temporal level, there's the good old-fashioned Unintentional Period Piece. A work or a musician can sometimes wear too much of the time of its popularity on its sleeve, to the point where people in later years find it difficult to appreciate beneath all the dated fashions, music, and pop culture references. This can be especially so if key plot mechanisms, lines of dialogue, jokes, gameplay mechanics, or lyrics can't work in later years due to changes in how the world works, leaving future audiences without the cultural reference points that people at the time might have taken for granted.
One common way this happens is for a franchise to do something that is widely rejected by the established fandom and fails to allow it to pick up a new audience. Falling victim to The Chris Carter Effect is one of the easiest ways for this to happen, as fans' memories of earlier seasons, books, films, or games become tainted by the realization that the plot that they had spent years following is going nowhere, is being made up on the fly with little forethought, and isn't likely to be resolved. Consequently, the now-former fans tell newbies not to bother. Another way for this to happen is to try and catch younger audiences by using fads, which can leave longtime fans outraged and fail to bring in the new audience that was hoped for. Finally, Darkness-Induced Audience Apathy may leave fans with nobody to root for, as the Eight Deadly Words start to take over. In a nutshell, the work enters a Dork Age that it never gets out of, Jumping the Shark so badly that it rubs off on when it was still good.
This goes doubly so if there is a Franchise Original Sin involved. Fans may look back on earlier installments and notice that the seeds for the Seasonal Rot were there all along, but simply had not flowered yet or were being held in check. If this happens, memories of the work's Glory Days may be tainted even further, as it becomes clear that things were going wrong right from the start, and that it was probably doomed to turn into the mess that became later on.
Ending a work on a base-breaking note is another way to do this. A work may be unable to tie enough plot threads or may not have an ending at all. The work's ending may be considered overtly mean-spirited, mind-boggling, nonsensical, predictable, pointless, or just unimpressive, which ends up turning off former fans and any potential new audience members.
Alternatively, It's the Same, Now It Sucks! can come into play. If a creator's output or a long-running franchise starts to grow stale, fans can turn against them, seeing them as having run out of things to say or do. In extreme cases, fans can turn against earlier installments from the creator or franchise as well, seeing the work they once loved as having always been lacking in creativity once the mechanisms of its Strictly Formula nature became apparent. This is less likely to happen if the formula was part of the appeal from the beginning; for example, Angus Young of AC/DC, a band that built its entire identity around simple, old-fashioned hard rock to bang one's head to, once proudly joked about having "put out twelve albums that sound exactly the same." However, if a key part of a creator or work's appeal was that it was innovative or revolutionary, watch out.
Either way, people came to see the work as overrated.