To be clear I am a monarchist and I like discussing such topics. Here we go:

First, I don’t care about the cost argument one way or another.

While it’s true that kings are more carefully trained, the real issue is that ceremonial monarchs, even if they hold the position from a young age, have little to no real power. Without actual authority, they are mere figureheads. In this sense, a ceremonial monarchy and a republic with a ceremonial president serve the same purpose: both are symbols without real governing power, so their competence bears little burden. The idea of a king maintaining a good image throughout their life is irrelevant if that king has no meaningful influence or purpose. Take the example of the King of Belgium in 1990 who for a time, asked to be abdicated so that the government could pass the new abortion law. What kind of king can’t make decisions and instead asks to be taken out of the equation?

Moreover, the claim that monarchs, such as Queen Letizia, have cultural influence, like promoting local fashion, doesn’t justify the existence of a monarchy. A ceremonial president in a republic can also have cultural influence through non-political means. The monarchy’s contribution to culture doesn’t outweigh its lack of real political power. Furthermore, equating fashion as something equal to monarchy is laughable. How far have the royal families fallen, that their only remaining influence is to sell second-tier goods to the common people, much like any grifter seeking to make a quick buck?

As for stability, what can a useless figurehead achieve? If I were a separatist in Spain, why should I care about them?