When Thailand's Crown Prince Maha Vajiralongkorn became the new King last week, he became the 10th monarch of his line.

His predecessor and father King Adulyadej Bhumibol, whose 70-year reign was the longest among his contemporaries, was widely loved by his people and his October death plunged the nation into mourning.

While the idea of inheriting a birthright to rule may seem outdated, there are still 43 countries around the world that recognise monarchs as their heads of state.

Since King Bhumibol's death, the title of longest-reigning living monarch has now passed to Queen Elizabeth, who took the throne in 1952 and is the head of state in 16 Commonwealth realms — including Australia.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 30 seconds 30 s Thailand's Crown Prince accepts the invitation to become King.

Associate Professor of politics and international relations at the University of Notre Dame John Rees said monarchs — even those that exercised political power — were no longer omnipotent.

"There are no state sovereigns in the world that hold power without some sort of relationship to parliament," Associate Professor Rees said.

"The monarchies of the Middle East, such [as] in Saudi Arabia exercise considerably more power than a prime minster or president would. But even here there can be strong accountability to the parliament."

"Monarchs wielding absolute power without accountability seem long ended."

He said some monarchies had endured as their sovereigns continued to play a central role formally and culturally.

"The newly crowned King Vajiralongkorn of Thailand is a good example of this," he said.

"Others remain without political power but will endure as symbols of national unity, such as the Emperor of Japan.

"Monarchs can play a spiritual role for people, almost as an expression of state-sanctioned religion."

The late King Bhumibol Adulyadej of Thailand enjoyed immense popularity in his home country. ( AP: Wason Wanichakorn )

Australian support for Monarchy highest since 1990

Luke Mansillo, a PhD candidate at the University of Sydney's Department of Government and International Relations, authored a paper this year which showed support for the monarchy in Australia was at its highest level since the 1990s, when a referendum on whether Australia should become a republic failed.

In 2013, 47 per cent of the population surveyed supported the monarchy, up from a low point of 34 per cent in 1998, but still down from a peak of 60 per cent in the 1960s.

Support for a republic has also declined — in 1998, 34 per cent believed Australia should "definitely become a republic" compared with 26 per cent in 2013.

Mr Mansillo said the generation of Australians who were in their teens when Labor prime minister Gough Whitlam was dismissed in 1975 and were also exposed to scandals in the royals' personal lives in the 1990s was more in favour of a republic than their children.

"When the dignified part of state — all these people who are supposed to be the best of the best — are no longer dignified, it's reasonable to think poorly of that," he said.

"But in recent years, the dignified part of state has been dignified. There haven't been scandals in any large sense with either William or Harry. There's no reason to problematise the monarchy."

He said most younger Australians saw other issues, such as same-sex marriage, as being more important.

"The notion of a far distant person being our head of state is not really bearing on the lives of anyone in any material way," he said.

What's the appeal of a monarchy?

Writer and historian Dr Aron Paul said the secret to the success of the British monarchy was that they had changed with the times.

The royal family is more accessible to the public than ever before. ( Twitter: Kensington Palace )

"The meaning of the monarchy has changed over time to fit in with changing social and political values," he said.

"[In ancient times] they were supposed to be extraordinary individuals who were special because they were chosen by God.

"Now with democracy they've found a new way to legitimise themselves … based on this idea that rather than being extraordinary, they're really ordinary."

The British royal family now routinely shares intimate family photos and videos on social media, and Prince Harry recently took the unusual step of confirming his relationship with US actress Meghan Markle in a bid to protect her privacy.

"People are interested in their personal lives and relate to them as individuals," Dr Paul said.

Will we ever have a world without royals?

"At the moment, you wouldn't put your money on [the demise of the British Monarchy], you'd say it would go on forever," Dr Paul said.

"But that's part of its mystique — the idea of an institution going on from the ancient past into the unforeseeable future."

Mr Mansillo said there were other, more technical reasons Australia was still a Commonwealth realm.

"If you like accountable government, no-one's made a better model," Mr Mansillo said.

"You have to pass money bills [budgets], votes of confidence … there is a minimum level of good government that needs to be maintained, and the conventions allowing this to exist are difficult to imagine in a Westminster system with a president."

"It's fair to say you're against something — but what will you do as an alternative?"

But Dr Paul warned "anything can happen".

"The problem with monarchies is you're only ever one or two lunatics away from destruction," he said.

"But you can say that about democracy too."