You're right, it's not the whole of it, but it's a start I think. I'm willing to try anything within my control short of mods and cheating to keep my town going
Plus, if you have all the original inhabitants make babies, and you control who falls in love with whom, so long as you know a sim, you can always invite them over, and the gene pool for future generations will keep going.
If you age up an enscorcelled sim, do you get to chose a trait? I suppose if you enscorcelled a future spouse you should randomise the trait, but if they're not a future spouse you could give them 'family oriented' and that could also help the town population along. Maybe.