Of all the arguments against monarchy, this is the stupidest. What difference does it make what century it is? Spain's monarchy is an integral part of the country's heritage, and a central part of its culture and traditions. To do away with it would be madness.
Let's look at the record of countries that have done away with their monarchies, as you so foolishly suggest. I'll stick to Europe in order to keep this short.
England (1649). Puritan military dictatorship, the prohibition of dancing, music, and theatre, and a genocidal campaign against Irish civilians. Fortunately it only took the English 11 years to realize they'd made a horrible mistake.
France (1792). The Reign of Terror and 23 years of devasting continent-wide war.
Russia (1917). 74 years of Communist tyranny, with some 20 million murdered under Stalin alone, not to mention the Cold War and all the real wars it spawned.
Germany/Austria (1918). The Nazi dictatorship, World War II and the murder of 6 million Jews and millions of others. Yugoslavia, Albania, Bulgaria, Romania (1945-47). 40+ years of Communist dictatorship.
I could go on, but I won't. It ought to be clear that abolishing a monarchy is about the worst thing a country can do. Any conservative, even an American, should be able to see this. But these are only negative reasons. A monarchy provides a non-partisan, apolitical focus for patriotism and a living link to a country's past. Royals perform invaluable charitable services which brighten the lives of countless people. A monarch who inherits his post is more likely to be a decent person than a politician who must scheme his way to the top. These are only some of the reasons why I not only hope that Europe's 10 existing monarchies will last forever, but that at least some of the continent's republics will come to their senses and bring back their kings.
And just look at it.