i consider myself a "gamer-at-heart" altho im far from being a pro gamer etc    so i try my best to stay away from involving $$ in any games i play (apart from the initial investment of buying the game itself) but hey to each their own right 
i hv on few occassion almost succumbed to the temptation of buying gold from 3rd party knowing that i can get it cheap & can easily make back the money i used to buy gold by flipping GAH > RMAH like most of flippers do but as ive said in above statement, it defeats the real purpose of me playing the game not to mention its against Blizzard's ToS so its the same as cheating using a bot   
There's no harm @ wrong on making some side income while u play but if the making $ overshadow the gaming experience itself (indirectly affects other true gamers who grind n save gold just to try buy good gear only to be sniped last minute by some flippers .. i truly understand the frustration) might as well spend the time & knowledge of buy low sell high in a more profitable environment eg forex trading etc 
Lately theres been a lot of talk about few pro d3 players (kripp .. alkaizer cough cough) b****ing about how the game is doomed bla bla bla .. they rushed to get to end (para 100)  equip their toons with BiS gears (which im sure with some help of $ one way or another) and now they're left with nothing to do    i keep hearing ppl saying D3 is dying etc etc from months back YET each time blizz comes out with new patch they come back in droves n start QQing again    IF indeed the game is dying i wouldnt be seeing 10k players just on PTR & so many more on live server .. to those player who still enjoy grinding to get their own gear upgrades & patient enuff to wait for whatever future upgrades/patch Blizz hv in store, they will always be here .. maybe those who say the game is dying r those who used to make $xxxx per month from RMAH but now only able to make a fraction of that 
in short a game lifespan is not determined by how much money u can put into it @ how much money u can get out of it BUT how u enjoy playing it over n over again .. just my 2 cents