I understand that it is not as easy as creating a new channel for everything, however there are a lot of breeding posts that can also be done by catching and the only difference would be the level, and if the breeding channel didn't exist, all of those commissions could be done just as easy in the trading chat. And as for the moderation, the Breeding channels are not that active so I would think that the same people could mod the proposed channel, and if it turns out it is too big for them I think it would mean that the channel is popular and was a welcome addition.
By opening those channels it created a "job" for those who like to breed, and a hunting channel would do the same thing for those who like to catch and battle, giving an extra incentive to EV train by battles, farming raids, shiny hunting in the wild, etc. because then you would get the rewards of those activities and the rewards of potential jobs posted in the channel.
As a last point I'd like to bring the Title Power sandwiches, a lot of people just see them as a side bonus or even just ignore it since all they want is the Encounter and Sparkling power, however if you accumulate multiple compatible jobs (lets say 3 just like the breeding channel) it would be way more cost effective to create those sandwiches and it wouldn´t take that long.