I love milk.
The reason specific farm subsidies exist when they're not truly needed anymore is because it's very hard to end popular programs in politically influential places. Most people, liberals and conservatives, like farms, and farm subsidies are broadly popular across the political spectrum. It's one of those issues that gets play in fairly niche political circles, libertarians and very ecologically focused people, but broadly, it's kinda like "eh, no farms no food right mate?"
If the government has to buy it to prop up the industry, doesn't seem like a successful industry
Not really a good rule. The alternative energy industry exists because of government incentives. It's becoming more viable, but only because those incentives subsidized growth in nascent industries, and this is a common, repeatable pattern. The internet is obviously a viable business and consumer platform today, but it wasn't for about the first 30 years of its existence, it was entirely funded by the American government and developed by government-funded (or tax incentivized) universities. Most silicon valley tech companies, even if they weren't directly subsidized, were frequently the product of Stanford, the Hoover Institute, military financing, and other public/private partnerships. I definitely wouldn't have my job as a software developer if it wasn't for government subsidies propping up this industry for ever.
It's part of what boils my piss about Elon Musk complaining about paying taxes. It's like, ELon, you fuck, the reason your company is the most valuable auto company in America is because *the federal government has subsidized every aspect of your business*