I know my name is not in this, but if I were to hazard a guess, so long as nothing wacky happens or the line of Louis XV doesn't die out like it did IOTL, I can see the Kingdom of France under the Bourbon dynasty survive for much longer than it did. As for French demographics, it is possible for it to recover sooner than it did IOTL, because in IOTL as you said, pre-Napoleonic birth rates were not achieved until 1937, but what probably didn't help was the various major conflicts that France had to go through, and that included 3 revolutions (1830, 1848 and 1870) and two major wars IOTL (the Franco-Prussian War and World War I) that probably exasperated matters.