Admittedly the days are gone when trade houses had a free hand to manipulate the futures, but then those times never really existed in the first place. There was usually another trade house of equal size willing to call the other one's bluff. Besides, markets are efficient enough to ensure that manipulations are short-term affairs. Trade house access to finance has recently been reduced, but that could best be put down to the inability of the trade houses to price risk correctly. The fact that the banks are no longer willing to sub-contract their role to the trade houses suggests that the trade houses have not filled that role successfully.
Bankers are a fickle lot and different areas of lending move quickly in and out of fashion.
The commodity trade is now out of fashion but that won't last forever. A scarcity in the supply of finance will eventually push differential rates higher to a level where risk is correctly priced. At that time, banks will be back knocking at the door. Finally, the low world price is a problem, but not just for the trade houses. Pre-financing quickly turns to dust when the world price falls below the cost of production.