There are several ways of "seeding" a town without mods, and at least one method of getting "new blood" into town which does not require any use of cheats or edit town. It would be nice if this aspect of story progression was fixed, but, somehow, at this late stage, with Sims 4 soon to be published, I doubt if it will be fixed.
If you need a few extra teenagers in town to provide romantic interest for your teens, then you can invite the newspaper boy/girl in and give them a birthday party. If it's a young adult you need, invite them back the following day and give them another birthday party. You have an endless supply of "new blood" this way and it is a valid method for challenges etc.
If the game you are playing is simply for fun then, the method I use is to produce a number of pregnant families and save them to the bin. To do this, I started a new game in a town I never use for normal play, and in turn, I made each of the townie couples my active household, used the cheat to get them both fertility treatment then...... Finally I exited to edit town, evicted them and saved them. The joy of this method is, you can be sure of at least a single birth when they have been placed in town, but, more often twins or triplets and you have no idea what sexes the new borns will be. Once I had enough pregnant couples in my bin I simply abandoned the game.