Without concretely proving it, antinatalism appears to have the framework to be correct, without having all the concrete required to fill the framework:
1a) There is always a negative affect before a positive affect, in regards to all humans as a whole (not necessarily to individual humans -- one person's gain can come at the expense of others). Happiness, pleasure, joy etc. (positive affect) are the result of relieving oneself/people of sadness, pain, sorrow etc. (negative affect).
For example, we don't feel relief in drinking unless we were previously thirsty. You can drink without being thirsty, but you won't experience positive affect in that. It's the preceding negative affect which allows for the creation of the positive affect.
Furthermore, when someone is relieved, boredom slowly creeps in (which is negative affect). So, even if you achieve your goals and relieve yourself of the negative affect, you'll soon be hampered by boredom! And you know what is required to counteract boredom? A goal (i.e. something that produces negative affect). So, both achievement of goals AND working towards goals creates negative affect (and only offers the chance of positive affect).
You can further compound this negativity by the existence of comparison, of which makes positive affect have a limited ceiling. In other words, because you can compare your lot in life to others, even if in isolation you would experience positive affect, you may experience negative affect if you see your lot in life as less than others (and will be particularly aggrieved if you deem this difference unfair). An example of this: people prefer being objectively poorer if they are relatively richer compared to those around them -- not everyone can be relatively richer than everyone else around them Is more always better?: A survey on positional concerns - ScienceDirect
Thus, negative affect is guaranteed whereas positive affect is not.
Therefore, there will always be more instances of negative affect experienced than instances of positive affect experienced.
The question now becomes: despite having greater instances of negative affect, do they outweigh the fewer instances of positive affect?
1b) Per unit, negative affect outweighs positive affect
Albeit, this is an exercise in estimation, as we cannot (currently) measure and compare ever instance of positive and negative affect to have been experienced by humans (it's framework without the concrete).
But, nonetheless, we can estimate.
What do you think of drinking water? If you could flip a coin to (heads) experience your quenched thirst or (tails) to experience dehydration, would you flip the coin? According to current loss aversion theories, people would prefer not to flip the coin because they don't like risking their status quo Higgins_et_al-2018-Journal_of_Consumer_Psychology.pdf (columbia.edu) . In fact, people would be far more willing to take risks to avoid dehydration, than they would to gain a more pleasant drinking experience (perhaps a sugary beverage instead of water). Hence, we have the backing of research to suggest that per unit, negative affect outweighs positive affect.
Since per unit, negative affect outweighs positive affect, and from argument 1a we saw that there are more instances of negative affect, it follows that there is a larger value of negative affect than positive affect in life, thus human life shouldn't be brought into existence.
2) Humans are totally unable to consent to life. This problem is amplified by the entirety of the arguments above, but it's also a problem in itself.
It's morally questionable to impose upon people serious conditions (in this instance: life) without their permission. Even if you think it's a good idea, and even if the life ends up being a good one, there is guaranteed risk involved (e.g. cancer, disease, childbirth complications, death etc.) which parents impose without consent.
It would be like someone using your money to go to the casino. Would you like it if someone did that? We already know they're playing against the odds (see my above argument 1a + 1b). Even if the person were to double your money, they still used your money without your permission. They still risked your money without asking you first. This is analogous to bringing humans into existence -- no consent beforehand.
Thus, parents gambling with the wellbeing of others without their consent is immoral. Therefore, childbirth is immoral.