I think these things usually come down to perceptions of human intervention as basically bad. If you actually witnessed the death of every individual dog in your scenario as compared with euthanasia, it's not clear that it would be more pleasant at all. These dogs will die diseased and malnourished by the roadside. Again, I am not really at odds with your emotional position, I'm just interested in the moral foundations on which we sit, if that makes sense at all.

If I may make an attempt at the moral underpinning, it is something like "survival as intrinsic worth". There are flaws to this position, as with any.