Being mean to someone who is not themselves being mean or manipulative is often not just counterproductive and self destructive (due to the reasons you mentioned), but also the result of personal weakness and lack of control. Meanness usually results from one of the following situations:

1. We feel angry and speak impulsively, entranced by our emotion.
2. We speak carelessly and don't realize the potential emotional consequences of what we are saying.
3. We are consciously and knowingly trying to hurt another person.

In case 1, our anger reflects a personal weakness in that our emotion prevents us from behaving level headily and making sure that what we say really promotes our interests. In case 2 we are speaking without awareness of the consequences of our actions, and hence again put our interests at risk. Only case 3 has a shot of being (at certain times) a good strategy, but in that case we must ask why we are consciously and knowingly choosing to hurt another human being, and whether such an action is ethical and justified.

Unfortunately, the arguments that are most convincing to human minds are often not the most logical or the best supported by evidence. To be as convincing as possible, one must appeal to the emotional, as well as the rational aspects of the brain. Arguments are unlikely to succeed when your audience is put on the defensive or made to feel as though their world view is under attack, since this will trigger emotional states. Anger, annoyance and resentment affect the proper functioning of our logical abilities (think of what happens when you try reasoning with a person who is upset, or of the stupid decisions made in "crimes of passion"), and hence will damage the effectiveness of your argument. When discussing controversial topics, it is important (though quite difficult) to make your points without emotionally arousing the reader. Hence, one reason that it is important to display niceness in your writing is that it makes it less likely that you will annoy your reader.

Atheism has some properties that religion does not that may allow it to spread rapidly under certain cultural conditions that likely will exist in the future. For example, as technology continues to play a larger and larger role in our lives (and continues to spread to the poorest countries), that may well correlate with an increase in the respect for and interest in science that people have, as well as the number of people trained in scientific fields. As science tends to directly contradict many religious stories (such as the creation of the world in Genesis), and since levels of religious belief among scientists are generally much lower than among the population at large, that may increase the rate at which atheism spreads.

Your argument essentially amounts to the following:

1. Melatonin significantly improves sleep quality.
2. It has no side effects.
3. It has low cost.

If all of these are true, then who wouldn't want to take it? However, you spend a lot of time on discussing point 3, but little on points 1 and 2, which are arguably the most important. How do you know that Melatonin really improves sleep quality so much? Is it just based on your personal experience (and perhaps that of other people you know)? If so, that is not convincing, as large scale randomized controlled studies are generally the only way to reliably tell if a medicine works. There are too many complicating factors like individual differences between people, the placebo effect, random fluctuation, reversion to the mean, difficulty in remembering how we felt in the past, etc. to rely on anecdotes.

Another point that your article does not address is the fact that there is a difference between a medicine having no known side effects, and a medicine ACTUALLY having no side effects. Any time that you take medicine you are taking a risk of a reaction that is unknown, or which failed to be uncovered in any studies that were done on it. For example, it is probably unknown whether a decade of Melatonin use (rather than just one or two years) causes problems of any kind. This sort of danger is unfortunately difficult to quantify, but I believe deserves at least some mention.

It is not a good idea to try and predict the likelihood of the emergence of future technologies by noting how these technologies failed to emerge in the past. The reason is that cryonics, singularities, and the like, are very obviously more likely to exist in the future than they were in the past (due to the invention of other new technologies), and hence the past failures cease to be relevant as the years pass. Just prior to the successful invention of most new technologies, there were many failed attempts, and hence it would seem (looking backward and applying the same reasoning) that the technology is unlikely ever to be possible.