'When' the war is won is the key element here. An early victory during the fall of 1914 will only serve to validate the more autocratic elements of Germany, Austria and the Ottomans. If the war is won in 1918 or 1919, and all three empires go through major civil strifes, democracy becomes a matter of time. It will come either through revolution or reform.
Germany and Austrian Cisleithania most likely last to present day as constitutional monarchies. Specifically in regards to Austria: Without another Ausgleich in the 1920s, this time to ensure representation for the Czechs, Slovaks, Croats, Slovenes and maybe even the Ruthenians, there's very little chance that it survives as the Empire which won the Great War. As to the Ottoman Empire: there are too many unresolved conflicts in the region (religious and otherwise) that I find it very unlikely to survive the entire 20th Century. But then again, there are so many butterflies by the time the war is won, that a unifying figure might just come along during the 1920s or 30s and finds a way to keep it together.
The Germans and Austrian emperors remain as heads of state, perhaps holding more power than the ones in today's western democracies, but as symbolical figures nonetheless. The Ottoman Sultan is another matter altogether. I'll defer to someone more knowledgeable.
Finally, in regards to the Central Powers as an alliance: as long as there is a credible threat to all three empires, let's say, an industrialised Soviet Union, the alliance keeps making sense. Governments come and go, however, so it's difficult to say for how much longer the alliance lasts.