When we talk about implementing breeding in Pokemon Go, Niantic does not do anything about that. We think that this option is necessary for the game because it could make the older Pokemon available for the new players that will come.
Breeding is compulsory to get Gen 4 mythical Pokemon Phione. In the series, this Pokemon can be made by breeding Manaphy, another mythical. Another thought, it would be exquisite if Niantic decides to put Phione as a Raid Boss.
If this option becomes available in the game, when the first member of the species desired is caught, we wouldn’t have to spin Pokestops in order to get the right km egg. To spin stops for eggs would be an option if we all are looking for super rare mons. But maybe, this would be a threat to the game’s profit, because we would need fewer eggs to catch. But, the players would be more vehement about eggs, thus leading to more incubators being bought. The IV-obsessive players will breed and hatch a lot until they get a 100%.
And as always, Niantic will always find a way to restrict the way breeding works. As a suggestion, they could limit the number of eggs obtained from breeding, which will lead to us spending more resources, like Stardust or candy just to produce an egg. Or maybe this could make us interact with gyms and walk given distance. And of course, Legendaries and Mythical should be non-breedable, except for the Manaphy/Phione case.
When this process is done correctly, it could become a vital part of trading. So in order to get something, you can only get it in egg form, which you need to hatch and hope the IVs are good.