The Spanish modern weakness only proves the Carlists right.
Most of human history, the monarchy has been depraved -- even Catholic or lutheran or Calvinistic or Anglican or Moslem or Hindu etc
The best form of government is a republic during peace times and a dictatorship during war. This was clearly apparent in the Polish-Lithuanian commonwealth where the King was powerful but not all powerful and while he could not be deposed, he was elected by the barons
I would not want Obama as King. I would not want most of the Spanish monarchs as king either
the Spanish weakness is more an indictment against welfare state policies (which are intrinsically leftist) rather than about monarchy or not
A constitutional monarchy has its good points, but a despotic monarchy has none right now. It may have worked in the 14th century but not now.
A rabble-rousing democracy is not ideal either, but a pure republic makes more sense
Even a democracy but a pure democracy like in Switzerland where people vote on every matter can work, but you need to be very small for tha.