There is no issue, it's a feedback loop.
If the market crashes, then clearly killing mobs becomes more profitable. Items are farmed less as more Gil enters the system which creates a situation where demand will begin to kick in.
If the market flourishes then it might be more profitable to sell items. The demand is slowly matched and profits will begin to diminish.
It's not a binary process where you must pick one or the other, they both work with each other to form a system. Granted, this system isn't as smooth flowing as I just described, but those who understand (and somewhat control) this system will easily make profits. The only way an issue could exist is if one or the other aspect of this is completely ignored, and this isn't going to happen. After all we are gamers, So if it's profitable we will use it! That's the beauty of it all.