This, and again, they planned to have half a million people go there by the 60s. People overestimate how populated Libya was during this time period, how many Italians already where there, and as you said how poor South Italy was. Many Italians already where moving there en mass, and many more would if Italy retained it for longer and didnt enter WW2. A much more reactionary Italy (which a surviving Fascist Italy would be) would also no doubt have higher birth rates, and in a settler colonial environment the government would likely actively be encouraging the Italians to have larger populations. Also there where more opportunities then just petroleum, hence why so many Italians moved there in the first place before much of that was discovered. I also keep seeing Democratic Italy being brought up long term, but even assuming that happens (which is not guaranteed, especially when the West is likely to support Italy as a ally against the Soviets), and it happened in say, the 60s, that would still leave over two decades for the government to actively encourage tons of Italian settlement, which was ALREADY STARTING TO OCCUR when WW2 began. Besides which, Libya was considered the "fourth shore" and an inherent part of Italy, or at least that was the idea that was attempting to be ingrained, and surviving Fascist Italy, even if it eventually becomes Democratic, would have far more time to build that idea up, and the Democratic government would not necessarily be eager to give up a territory that is full of Italians and that has been built up for decades as an inherent part of Italy in the public consciousness. France was a democracy, and it still fought extremely hard to hold onto Algeria because it was viewed as an inherent part of France, Italy would likely do the same with Libya even if it did go democratic, and considering the far lower population of Libya compared to Algeria, is likely much more successful.