To be blunt just because you don't know anyone who keeps livestock in the town world doesn't mean that no one does, it simply means that you have yet to hear about it. Absence of a thing does not mean the nonexistence of a thing, and vice versa.
Onto another note, when would the console be doing this? How often? As soon as it hits cap? No matter what you put it at you will always run into it screwing someone over. What if someone was at a sheep spawner and had just dyed all the sheep that had spawned with lapis and then the console did /killall? A waste of their lapis, and I can assure you if that happened to me I would be more than irritated for such a thing to happen.
Or if someone is in a dark room with wolves they'd just spent a lot of time either breeding or taming at a spawner? Again, all their work would be wasted.
As Duel was saying, I definitely remember when this type of thing happened in V5 and I remember back then not being very happy that it kept happening. I don't think it should be implemented, this should be resolved in a manner that's not this.
In addition to be honest it didn't start when spawners were added, it was happening far before that. It's one of the reasons why leather was such a high price, because mobs wouldn't spawn due to the limit being hit. Spawners being added didn't make the issue suddenly occur it simply made it more obvious than was before.