I usually force myself to quit when Im playing the game for the threadmill addiction instead of fun. This was so i.e in Eve and WoW.

My first "mmo" was zombiemud. After playing it year and half I realised I had never had much fun in the game. The main thing I remembered was sore eyes from all the hours playing a text-game while running in super-thin threadmill.

Id say for newbies of threadmill games the game is first both fun (because its new experience) and addicting. But with time the fun part goes away leaving only the latter. After that they expect somthing extra from new threadmill games. That extra could contain sidefeatures like story or/and improved threadmill.

Still Im sure there are players who never gain back the fun from improved threadmill alone. They will never again play a pure threadmill game. They simply burned out on the first run and lost the fun forever. Their taste was refined.