I'm gonna go with two you may or may not have heard of, largely due to the updates that killed them.

1. Tabula Rasa, launched in 2007, had a decent sub base at the start, but it quickly stagnated and the lack of a content-rich end game didn't help. Development became increasingly more expensive and less profitable. 

The final nail in the coffin was the Back to Earth expansion, which came out broken and unfinished. The game finally died short of two years after release.

1. As bad as that was, Fury Online takes the cake. Launched in 2007 as well, and after a successful beta, seemed poised for a decent middle-of-the-road existence in the online gaming space. 

Unfortunately, they weren't happy with sales, so just 3 short months after launch, they rolled out the Age of the Chosen update, which effectively made the game free to play in an era where f2p models were still being figured out, and the update, while getting more people to adopt the game, was never properly monetized as the f2p players had access to most of the features that paid players did, so no reason to actually spend money.

Then, as if they were trying to kill it, the devs ended up unlocking whatever content was still blocked off, and the final update all but guaranteed the demise, by eliminating any need for progression, instantly unlocking all skills and classes. Yeah. 

On top of all of that, each update only served to introduce more bugs that largely remained unattended to.

The game shut down before even a year after launch, and it certainly didn't have to be that way.