If there is not democratic reforms, the shit could hit the fan very badly. Franco's last years were not pleasant, with the opposition progressivelly stronger, more active and more audacius. Industrial workers and students became a pain in the ass for the regime during the late 60's and early 70's. Even the traditional falangist stronghold, the Vertical Unions, had been infiltrated by the opposition through the Comisiones Obreras (Worker's Comissions) You had also active armed groups, like ETA (who killed Carrero Blanco), MIL or FRAP and after 1975 GRAPO. Meanwhile, the international economic crisis had its worse effects over spanish economy in the late 70's, the unemployement and inflation skyrocketed, and Spain became the european country with the largest number of strikes and social conflicts. In the meantime, the regime had lost its traditional internal cohesion and there was an open political war between the regime's famillies, mainly Falange vs Opus Dei. Increasingly sectors of the regime considered that a democratic reform was necessary, both because their economical interests were aimed towards the european common market and because they feared a social explossion, specially after the Carnation Revolution. At this point, France and Germany were supporting opposition organizations activelly and specially the SPD was funding the PSOE, while the PCE had developed a strong and active network inside the country. There had been already contacts between the opposition parties and moderate elements from the regime in 1962, the so-called "Munich Plot" by francoist press (in fact, the expression they used in spanish was "Contubernio de Munich", that sounds rather comical because too bombastic. It loses in translation)
Those sectors inside the regime who wanted a democratic reform where the ones supporting Juan Carlos, since they understood that either the started a controlled demolition of National-Catholicism so they would keep their power quota or they would fall with the regime. In the early 70's it was clear that spanish society demanded democracy. In fact, there are already polls in the mid 60's made the FOESSA Fundation that showed an overwhelming support for a democratic republic. The arrival of european tourists in those years and the opening to the outside world that it brought, the economical development and the creation of an urban middle class, not to mention the mass emigration from rural areas to cities conforming a large urban proletariat, sometimes living with very poor services and very bad conditions regarding housing and infrastructures where also elements that dorve the change. Summarizing, people wanted to live like the frenchs or the italians and not under the thumb of priests and the boot of the army. So, as you see, it was much more than Juan Carlos what restored democracy in Spain.
So, a continuation of the regime would be extremelly conflictive, and the conflicts seen during OTL transition to democracy (that was not pleasant at all, including hundreds of deaths in demonstrations, strikes and acts of terrorism by both far left and far right groups) probably would be only increased. I don't think the country would fall in a civil war, however, since the trauma caused by the 1936-39 war was still very present and was a ghost haunting spaniards all along the political spectre. Nobody, or almost nobody, wanted another round of people being shot in roadsides.
And as possible sucessors, you have Luis Carrero Blanco as mentioned, if he had not been killed. Also, if Carrero Blanco still goes beyond our world, Carlos Arias Navarro, strongman of the ultras after Franco's death and PM before Adolfo Suárez could be a candidate. Probably we can find a large set of candidates, but I''ve not the mind for that at this moment.
