I do understand. I've read the OP and this quote several times. The flaw in the reasoning returns, and it's not about desire. The flaw is in the infinite amount of children and animals that have never been born.
"Should we grieve them? No." I agree. Grief is about losing something. If it does not exist, it cannot be lost and therefore grieved. But then a again, a mother may grieve for what she could not attain, an unborn child she desires to give birth to. However, the child is not floating around in a formless state just waiting to slip in when the sperm and egg meet. It does not exist at all. That is non-existence -- it has not come into being. Therefore it cannot even be envied, because there is no being to envy. It is not blissful nor free of desire because it has no form or consciousness. It is not even free, because it does not exist. The mother imagines the unborn child, desires it, grieves the loss of what she cannot pour her love into. That is what grief is, the loss of the object into which one pours love. The imagined unborn child becomes an object to the mind, but that does not mean it exists.
"They don't even desire positive things..." By calling them "they," by giving them, as the Buddha would say, name-and-form, it is saying they exist, that they experience, that they function. Therefore they can potentially experience not desiring. Something that does not exist does none of those things, has no form, and has no experience, cannot be named or labeled because there is nothing to attach the name or label to. Which may have been why the introduction of the non-number zero into mathematics was so revolutionary, because it made the non-existent come into awareness, and become something to which a label can adhere. That does not mean, though, that it has a form or an experience. Does it have potential? If it does, then it exists in some way, and therefore is not truly zero or nothingness. Can it be envied if it does not exist? Only because it has been given a label and therefore exists. Then it is no longer zero or nothing. It is something -- a product of the imagination, with no consciousness or form, and no experience, the same as the unborn child which a mother imagines, and the infinite number of unborn children and animals you imagine and envy. Envy is a function of desire, and it seems to me that the object of your desire is zero, nothingness that is an object of the imagination that can be labeled and takes on form, function, and experience.
I'm not being an asshole here. You asked for an opinion of your analogy, and about non-existence being preferable to existence. The objects to which you attach non-existence, unborn children and animals, are not objects, they have no form, experience, or state of existence, and therefore cannot be superior nor inferior, nor the object of envy as they are not objects. They are in a state of pre-existence, have potential, and can become, therefore they have a state and in some way do exist. They have and are composed of kinetic energy, potential. Therefore they are matter. I almost said they are inherently not objects, but that would be an agreement that "they," these labeled-yet-non-existent ones in some way exist, because only something that exists can have an inherent condition.
If, having existed, you find it is superior to cease existing, that I get. Suffering ends. Desire ends. But for something that never existed, it never began nor ended, and has no potential to do either. Therefore, imo, "they" cannot provide support for your argument regarding preference.