What if France's birth rate hadn't dropped below replacement level so early?

They say that North Africa, especially Algeria, would have become French territory by now.
However, France faced a population crisis in the 19th century as the first country in the world to experience a low birth rate.
While the population difference between Algeria and France is not large today, in the mid-19th century, Algeria's population was only at the level of a single French province, which would have allowed for the replacement of the indigenous population through large-scale migration.
In fact, France's true objective was not just to make Algeria a simple colony, but to turn it into complete French territory, and they intended to use the method mentioned above to replace the indigenous population.
The problem was that France's attempts at population replacement were limited due to a shortage of young people at the time. Ultimately, they were unable to carry out large-scale migration, and even during France's golden age, the French-descended population in Algeria accounted for only about 15% of the total Algerian population. Even then, they were all expelled during the Algerian War of Independence. (The famous movie "The Umbrellas of Cherbourg" is set against this background.)
It is said that if France's birth rate had not dropped below the replacement level so early, and if they had a large young population like the UK or Germany, there would have been a high possibility that most of North Africa would have become territory for people of European descent, much like the United States. After all, France's own population base was massive at that time.
It is said that France's premature low birth rate had a significant impact on allowing countries like Algeria and Morocco to continue to exist as they do today.