The Spanish Royal Family is now almost reduced to three people able to travel long trips, king and queen and now Leonor. Queen Sofia is now at a stage that she needs to slow down, but she still covers social events that are not so intense. And princess Sofia is not even ready until she follows the steps of Leonor, since if something happened to the king, she would be the new princess of Asturias.
The king's sisters are out of representing him since they are no longer working royals. Until Leonor and Sofia age a bit and become more involved in activities the heavy representation duties fall on Felipe and Letizia.
When I read comments from fellow Spaniards in some of the twitter links complaining on Felipe's trips, reminds me he is as head of state, and the queen, are covering same exact duties as a president and first lady in any republic. And no one ever complains.
But in the case of a monarchy the king and queen are not just covering representation duties but embodying a direct line from the king of Asturias that started to recover the peninsula, microstate by microstate over a millennia ago. No elected president or first lady can even come close to the historical relevance of a royal family's duty to the country their ancestors basically formed into one nation.
In private industries no one ever complains when the company passes from parent to child, they don't see they too copy the monarchy system at a family level.