The Prince of Wales

[Louis VI the Fat]

And I'd like to point out that some of the most stable, sane and long lived states today are monarchies: Liechestein, Monaco, Netherlands or the Vatican. In fairness if we were to follow an example of government, the Vatican's would be the one to follow as it has survived for 2000 years. As far as governments go, that's a success story in my book.

The Vatican is not a hereditary monarchy. It is an elected monarchy. As such, it is not really comparable with Liechtenstein, Monaco or the Netherlands. And distinctly incompatible with your preference for a massa 'who has been exposed to affairs of state since childhood'.
The Vatican's monarchy has not stable. It has been the subject of countless centuries of infighting between (the city of) Rome's powerful factions. Much like today's political parties, as it were.

Most elected monarchies have now dissapeared, because it is an unstable form of government. History. Most existing monarchies on the other hand, date from the 19th century:

The Netherlands started as...a proud Republic. This is what brought them their fame and wealth. The Netherlands has been a republic for longer than it has been a monarchy. The first ever monarch of the Netherlands was.....Napoleon's brother. That's right. Napoleon turned the Netherlands into a monarchy.

Monaco has been independent only since Monarchisms Great Century too. That is, the 19th century.
In the case of Monaco, 1861.

Liechtenstein is another 19th century monarchist product.

So in fact, precisely none of your examples are 'some of the most stable, sane and long lived states'.

Fair enough, so are Spain, Sweden, UK, Saudi Arabia, Australia, Canada, New Zealand, Belgium, Denmark, Norway. While some of the more unstable countries in the world are "democracies": Nigeria, Mexico, France, Sudan, Haiti, Colombia...

I agree that there are other factors at play here too, and yet the comparison is quite illuminating.

This comparison is rather fruitless. Leaving those seemingly trivial 'other factors' unmentioned too, I shall suffice with a more meaningful comparison:

Monarchy - Republic:

Sweden - Finland
Canada - United States
Belgium - Germany
UK - Ireland
Denmark - Switzerland
Swaziland - Zambia
Morocco - Tunesia
Thailand - Singapore

You see, Louis, it's not about being subjects, as that is inevitable. The difference is between trained masters and dabbling masters

Being a subject, a mere pawn of the mighty, is far from inevitable. I am a man not devoid of talent, and could well end up in a position of influence in my government. As could, and do, many others. I am not cut out for a 'you kind massa, me good boy' attitude.

Here's one of those trained masters, another Charles, who had 'been exposed to affairs of state since childhood' as you say, and to which an entire state made itself slave:

Spoiler Alert, click show to read: 

Charles was the only surviving son of his Habsburg predecessor, King Philip IV of Spain and his second Queen (and niece), Mariana of Austria, another Habsburg. His birth was greeted with joy by the Spaniards, who feared the disputed succession which could have ensued if Philip IV had left no male heir.

Pedigree of Charles II. Note large amount of inbreeding.

16th century European noble culture commonly matched cousin-to-first-cousin and uncle-to-niece, to preserve a prosperous family's properties. Charles' own immediate pedigree was exceptionally populated with nieces giving birth to children of their uncles: Charles' mother was niece of Charles' father, being daughter of Maria Anna of Spain (1606–46) and Emperor Ferdinand III. Thus, Empress Maria Anna was simultaneously his aunt and grandmother.[1] This inbreeding had given many in the family hereditary weaknesses. That Habsburg generation was more prone to still-births than were peasants in Spanish villages.[2] There was also insanity in Charles' family; his great-great-great(-great-great, depending along which lineage one counts) grandmother, Joanna of Castile ("Joanna the Mad"), mother of the Spanish King Charles I (who was also Holy Roman Emperor Charles V) became completely insane early in life.

Since about 1550 AD, outbreeding in Charles II's lineage had ceased. From then on, all his ancestors were in one way or another descendants of Joanna the Mad and Philip I of Castile, and among these just the royal houses of Spain, Austria and Bavaria. Charles II's genome was more homozygous than in an average brother-sister offspring.[2] He was born physically and mentally disabled, and disfigured. Possibly through affliction with mandibular prognathism, he was unable to chew. His tongue was so large that his speech could barely be understood, and he frequently drooled. He may also have suffered from the endocrine disease acromegaly.[3]

Consequently, Charles II is known in Spanish history as El Hechizado ("The Hexed") from the popular belief – to which Charles himself subscribed – that his physical and mental disabilities were caused by "sorcery". The king was exorcised, and the case of his exorcisms remains of the most sinister of the history of Spain.

Having learned to speak at age of four and to walk at eight,[2] he was treated as virtually an infant in arms until he was ten years old. Fearing the frail child would be overtaxed, he was left entirely uneducated, and his indolence was indulged to such an extent that he was not even expected to be clean. When his half-brother Don John of Austria, a natural son of Philip IV, obtained power by exiling the queen mother from court, he covered his nose and insisted that the king should at least brush his hair.[3]

An impotent, drooling imbecile. Kept on the throne for decades. This is what destroyed what was not long before the mightiest state in Europe.