Oh goodness, this conundrum...
Yes, Austria gets taken down early and Turkey may have difficulty expanding, but then anyone could make that argument that any country has its own advantages and disadvantages. Come to think of it, Turkey starts out very near to a supply-center rich Balkans, and once Austria survives through the early game its central position allows it to take any direction it wishes to take. Similarly, there are people who root for England and Russia is by some polls the most successful power, but Russia has a hard time concentrating its units initially whereas England requires a constant supply of convoys and a delicate balance between fleets and armies, something I have not yet been able to master.
So, take your pick. Turkey is not a bad power, persay. Yes, it has the nickname of "Sick Man of Europe", but as time passes players' strategies change. Maybe Turkey might be the most successful nation in Diplomacy again in turns of wins. Who knows.
Or am I just stating the obvious?