This is what happens when a crisis drags on and on, adversely affecting everyone's expectations of the future: ideas that used to be thought of as carping criticism, the hobbyhorses of cranks and trolls, begin to seem like reasonable propositions. Such is the case of republicanism. The monarchy has been losing the confidence the Spanish public used to feel in it. The Crown now gets about three out of 10 in the opinion polls, when its average used to be 7.5.
What happened? Well, aside from the rivers of pious ink that have flowed on the subject, the case can be stated briefly enough: the king inherited a Crown that - for anyone of modern views - was illegitimate. A quaint relic of the Middle Ages, fallen further into disrepute by its complicity with the extreme right for a century and a half, and in particular with the Franco regime. But by scrupulously correct behavior, in line with constitutional principles, Don Juan Carlos redeemed that essential illegitimacy.
In practical terms: as the dictator's demise drew nigh in the late 1960s, the extreme left (the Communists) at last accepted the deal - first made in exile in 1947 between the various monarchist groups and the moderate left (the Socialists) - that any transition to democracy would have to take place under a king or regent.
The fact that the king, and the government he appointed, carried out a substantial part of the opposition's wishes, explains why the legitimacy of the monarchy had so much to do with the person of Don Juan Carlos. It is commonplace that, without being exactly monarchists, a probable majority of the Spanish public were, at least, "Juan-Carlists."
For similar reasons, once democracy had been consolidated, a majority might well cease to be Juan-Carlists, at the least circumstance that produced a feeling of disaffection, drifting into overt hostility toward the monarchy. Such a drift is perceptible in the increasing numbers of republican banners seen in recent demonstrations by the left against government policies on matters as sensitive as healthcare, education, evictions and abortion. This is the chief danger of the monarchy's close connection with the person of the king: lack of confidence in one brings the other into discredit.
This is just what we have seen since 2008, in a process inversely parallel to what happened in the 1970s. If then the king's decisions gave legitimacy to the monarchy, now the conduct of several people, not only of the king but mainly of his daughter and son-in-law, have put public confidence in the monarchy under unbearable strain. And if in those days the legitimacy conferred on the institution by the king's actions made the monarchy/republic question seem irrelevant, it can now come as no surprise that the loss of confidence in the Royal Household has done great damage to the monarchy, and may make a republic look once more like a reasonable option.
To take serious note of this drift of opinion is the same as to suggest that perhaps the time has come to disconnect the person from the institution. The time of belief in the divine origin of royal power is long past, and no one now believes in Mother Nature as a code of conduct. Nothing is quite divine, and nothing entirely natural. The monarchy that now exists is there thanks to a deal struck 40 years ago. It would not be at odds with the essence of that institution if the Crown were held as a public office until a certain age, such as 75; after which the king would need only concern himself with the ceremony to invest his replacement as head of state. Like the pope, who, though owing his office to the inscrutable designs of Providence, is apparently enjoying a happy retirement.