In Spain I disagree that the monarchy is secure. It actually isn’t. No wonder they have to toe the line.
It would help the discussion if that view was based on something factual, otherwise it's just a statement without much meaning.
I think you confuse 'toeing the line' with being sensitive to regional movements and separatists who will use every move you make to their advantage. That's a delicate balance to walk for a sovereign, but one that His Majesty manages quite well, and is respected for.
Spain is a formal society, and the upper class, with the King at the pinnacle, still speak quite formally and they may seem detached to many. However, faced with the alternative, in a country where most dislike politicians immensely, there is no real worry that a majority would ever want to elect another politician (or reality buffoon) as head of state, instead of keeping the system they know, and that they have known for centuries and centuries.