Animal farming is like rain: the harm comes with something we need
Rain is not a choice, and life cannot go on without it. Eating animals is a choice, and large studies of people who don't show it is not needed. The analogy only works if necessity is assumed, and necessity is the very thing in question.
Plenty of things we rely on do damage as well as good, and we accept the damage because we can't have the good without it. Rain floods towns, but no one wants to end rain. Producing food for billions of people has always involved some animal suffering and environmental cost. Treating that cost as a reason to drop the whole system misses that it may be the price of something we genuinely need.
The analogy holds only if eating animals is as unavoidable as rain. It isn't. Rain is a natural process nobody decides on, and no living thing can do without water. Animal farming is something people choose to do, every day, at a scale they set themselves. Large cohort studies, including Oxford's EPIC work following tens of thousands of people, find that people on well-planned plant-based diets stay healthy. So whatever the case for rain, it does not carry over.
The second step also fails. Even where something is essential, its harms are not all inseparable from it. We don't abolish rain, but we do build flood defences and stop building on floodplains. The suffering of a farmed animal is not a side effect of feeding people. It is the method. Plants can be eaten directly, and they feed more people from less land than the same crops run through animals.
Welfare differs between farms, and those differences are real. They change how much harm is done. They do not make the harm necessary. On any farm, a healthy animal is killed early in its life, after separation and transport. That is not weather. It is a decision, and it can be made differently.
This is the same point as the general claim that suffering is inevitable: some suffering is, but that does not make this suffering so.
Treating two things as comparable when the differences that matter to the argument are large.
How it applies here: Compares animal farming to rain, an unchosen natural process that life depends on. The difference that matters here, whether the thing is optional, is exactly where the two come apart. Diet is chosen, and plant-based diets are shown to be adequate.