Villagers have no mechanism to maintain numbers.

The villagers should have a population control mechanism to make sure they maintain appropriate numbers, even with breeding and death from hostile mobs.

Currently they just breed at seemingly random intervals and die based on behavior. This can cause villages to become overpopulated or to die out. Most villages tend to die out if you stay with them in simulation range for an extended period of time, although the opposite could easily happen.

Summary:
Expected behavior: Villages should maintain reasonable numbers of villagers in each occupation based on statistics and maintain numbers.

Actual behavior: Villages spawn with a random assortment of villagers based on RNG, Villagers left to own devices. Populations can dwindle or explode at the will of the RNG and other factors. Villages can be killed off by ill behaved players on multiplayer servers and village will remain forever vacant.

Proposed solution (Clearly up to devs, but I thought's I'd give my 2 cents):
1. Breeding should be based on available space in village.
2. Villagers should have their occupations chosen based on combination number of villagers in each position, priority and RNG. (Not just RNG.)
3. In the event that a village goes down to fewer than 2 villagers, a cluster of villagers should be re-spawned after a random interval as a "final safety net.)
4. In the event of slight overpopulation villagers should breed less. In the event of severe overpopulation random villagers in overpopulated jobs should "forget to come in at night."