The second Spanish Republic was in charge for a bit over five years. In that time churches were being burnt, priests and nuns were being killed, there was a state of virtual civil war with daily murders, the leader of the opposition was killed by the state police, the rhetoric was that of a communist revolution modeled after the Soviet Union. That was the road to the future. And we don’t know what would have happened if they had continued in power? They may have suddenly turned into Mother Theresa? How much longer should one wait to find out? The Soviet Union took many decades of misery and repression before it fell. Pretty much the same thing in China under Mao. North Korea and Cuba are still holding out and just look at them. And in Spain which was headed down the same road people should have waited longer to see how things would turn out? Sorry but it was quite clear how they were already turning out. Clear enough that when the military lead the uprising millions of Spaniards supported them and fought against the Republic.
Franco died in 1975 and left a country with a solid middle class and a stable economy which allowed further development and democratization after his death.
To say the economy would have fared better had the republic continued until 1975 is… well, an interesting point of view. It would have been the one only case in history of a communist country doing well. Compare east and west Germany. It was not the communist side which fared better in any sense.