The First World War was really the last hurrah of the governing monarch, but they were in terminal decline well before then. Really they'd been in decline since the revolutions of 1848, where monarchs were slowly forced to give up more and more power to placate restive social movements.

We tend to consider Kaiser Wilhelm as though he were an absolute monarch for instance, but by the time he came to power there was a permanent German parliament with at least some power behind it, and concessions had been made to trade unionists to prevent the spread of socialism.

France had dumped its last monarch in the aftermath of the 1870 Franco-Prussian war, after two previous flirtations with republicanism. The monarchs of England had decisively lost their struggle for power against Parliament more than a hundred years previously. Other European countries had given major concessions towards constitutional monarchy to stave off republican or socialist movements. Russia had barely staved off revolution in 1905.

The pressure cooker of world war 1 really ramped up the stress Europe's states went through. Some cracked completely, like Russia, where two revolutions deposed and then executed their royals, or Austria-Hungary, where the country split along largely ethnic lines. Germany narrowly held off a socialist revolution. Even before the Treaty of Versailles, which heaped the blame for the war on the Kaiser, Germany was effectively under the rule of military dictators

But if ww1 hadn't happened, or if the central powers had won? The issues were still there, just left less exposed by the pressures of the war. Heavy concessions along federalist lines would have been required to even attempt to hold Austriahungary together, revolution in Russia was only a bad harvest or two away, and the Sultan in Istanbul was already little more than a figurehead.

Germany would probably have retained its King, but the drift towards constitutional monarchy would have continued.