What are the pros and cons of a sterilizing (~95%) plague?

You may think this means that only 5% of people, 1 in 20, will be able to have kids. But actually, it takes two, and if there's no way to predict who can and cannot, it becomes closer to 1 in 400. The 1/20 chance of the male partner being fertile is multiplied by the 1/20 chance of the female partner being fertile.

It's difficult to prove someone infertile, but if someone can conceive a child, that proves their fertility. This is of limited utility in the case of women, but I suppose a man of proven fertility could make a living as a stud, attempting to knock up eligible women. So once you've identified a population of fertile males, you knock that half of the equation back down to a 1/1 and the women can go back to the much better 1/20 odds. Of course, you'd need to re-identify potential studs over time from the newer generations as older ones die off. And unless each woman is having twenty daughters you're still suffering from rapid population decline and the attendant societal collapse.

So, in summary: It'd be real bad, and even if it didn't kill off humanity in a generation it probably would knock us back to the bronze age within a century. And even if we somehow manage to dodge that it will still change human society permanently in unpredictable ways.

Pros: No more people.

Cons: No more people.

...nope

A significant reduction in the population leads to better outcomes for the survivors. Mass death events in history, such as both world wars and the various pandemics, are usually (but not always) followed by a period of progress and prosperity.

Humans breed faster than their civilizations can keep up with and in the modern era scarcity is artificially created by arbitrary ownership of natural resources. Fewer people means more resources per person.

However if your hypothetical pandemic were to strike, it would create a problem the world has never encountered before (at least as far as I can find) so predicting what might happen is basically guess work. Given how violent brutes tend to be more successful in scarce environments, the outcome would be very grim.

Cons - lack of normal workforce replenishment ~18-20 years after plague would cripple economies, social safety nets and essential government spending (state pensions, road upkeep, military). Demographic would skew older and older with each year limiting democratic governments ability to pass any kind of rescue legislation until it's too late (assuming an increasingly older population votes in their own interest, being unable to work to help fix massive labour shortages, as so defensively protect government programs for elderly that government can no longer afford)

Pros - I mean, what counts as a 'pro'? Less pollution as the world economy collapses I guess..

Pro: less overpopulation in the long term.

Cons:

• more old than young people

• economy collapses

• society collapses

• we're done for basically

Con: Fertility value sky rockets resulting in either a society led by the fertile or led to force the fertile to breed. Ala hands maid's tale. Con: hand maid's tale

The world would be filled with old people.

You are thinking it would just be a gentle way to reduce population but even if women did artificial insemination to keep your 1/20 birthrate, the world as you know it needs a lot of people to run it - farmers, network engineers, maintaining all the systems that make your comfortable existence possible requires a baseline population with particular skills, of working age. There is a reason technology increased with population growth.

The natural world would be better off, probably. Humans would be in for a rough ride, and would probably die out.

What are the pros and cons of a disease that leaves people sterile? Uh.

Con: it's a disease.

Define plague?

You just see Children of Men or something?

Need more context

If it's a one-generation thing, you will see some (or perhaps a lot of) social upheaval and a lot of artificial insemination/stud services, leading to a relatively brief dip in the population. Seriously, look up when Earth reached 1 billion people, and we're closing in on 9 billion now. If it's every generation, humanity will go extinct. Each fertile woman would have to have over 40 children to maintain population levels.

Assuming overpopulation, food shortages, war, starvation, suffering of billions. Depriving people of the power to reproduce seems the kinder path.