The world is really complicated, and therefore, figuring out the best government policies is very difficult. Emphasizing simplistic ideological purity, to the point of applying one idea to all problems and ignoring the knowledge of people who actually study each problem, is a bad way to approach the world's complexities. I mostly think, in the U.S., the left has been doing a better job at avoiding the mindset than the right. But the popularity of Bernie Sanders doesn't look good in that regard.
I watched his last debate (and he's debating again now), and pretty much any attack he makes on Hillary is essentially about ideological purity - very similar to Tea Party attacks on "RINOs" that liberals previously mocked. And his own logic for whether to support something seems to always go like this: "Is this on the side of 'corporate America'? If yes, I'm against it. If no, I'm for it." Should I expect that question will work really well as a substitute for what actually matters: the net effect on the quality of people's lives?
One of the worst ways this plays out is his stance on trade. He, like Trump, opposes free trade and doesn't want American workers to have to compete with poor foreign workers. For one thing, support for free trade, and understanding how it is a net benefit to America, is one of the most commonly used checks on a person's understanding of economics. And besides that, we are seeing a rapid rise from poverty in the developing world in the countries that have been able to tap into globalization the most. The absolute poverty in the developing world is far worse than the "stagnating middle class" that Sanders wants to protect here. What Sanders essentially wants to do is block one of the most proven routes out of absolute poverty by forcing us to buy more expensive products from comparatively wealthy Americans. That is redistribution upward, from the poor to the wealthy in the world, and would increase overall inequality. But I suppose it's worth increasing poverty and inequality as long as we're opposing "corporate America" while doing so.
The same problem applies to immigration. Although he wants to treat immigrants who are already here nicely, he's actually anti-immigration in other ways, similar to his opposition to free trade. He has voted against immigration reform and explained that he does not want to allow more immigrants because they'll lower our wages and take our jobs. It takes a false, zero-sum view of economics to think that is true as its net effect, but suppose we give Sanders the benefit of the doubt here. The poverty of the immigrant that he imagines in these scenarios is worse than being a "stagnating middle class American" who he is trying to protect. Using immigration quotas to prevent poor people from earning a living for themselves here is again redistributing income upward, both increasing poverty and inequality (unless you don't count foreigners as people). But as Sanders has said, "open borders is a Koch brothers proposal", and using government force to increase poverty/inequality is just the price you have to pay to keep your ideological purity of opposing rich Americans I suppose.
Even though free trade and immigration are overall beneficial to our economic growth and toward reducing poverty and inequality, it is true that some Americans can be worse off. If Sanders wants only those exact Americans to be in his circle of empathy, there's a much easier way to help their incomes without shrinking our economy and forcing foreigners into crushing poverty. Just propose redistributing more income through wage subsidies!
Instead he wants to do redistribution through policies that sound terrible to me, such as "free" college (this gets at the main reason I think that's really wasteful), and a $15 minimum wage. Maybe a $15 minimum wage would work in a high-wage part of the country. But that is above the median wage in some of our states. How would it not be disastrous to tell a whole state "it is now illegal to pay anyone the amount that the majority of you are currently getting paid"? How can I understand his support for that other than as ignoring the complications of real life when they are inconvenient?
You know how liberals (rightly) attack the GOP for ignoring what experts such as climatologists have to say? Is it now OK to completely ignore economics? Because Bernie, for many more reasons beyond the issues I already mentioned, just ignores mainstream economics and chooses to believe what is convenient for his ideology. On Twitter I mostly follow economists and musicians - it's funny how whenever Bernie is mentioned, it's always negatively by the former group and positively by the latter. For example, past Democratic CEA chairs have openly criticized him for undermining the Democratic Party's reputation on evidence-based economics. You should laugh at Ron Paul when he talks about the gold standard because the overwhelming consensus of economists laughs at that. But be consistent. Bernie chose an MMT economic adviser, which is kind of like the left-wing version of that. Etc.
Does Bernie seem like a sincere, principled person? Sure. But that's hardly enough to make someone a good president. What matters most is the actual effects of what you do. If you make the world a worse place, I don't care if you did so sincerely, or if you didn't take money from super PACs, or whatever.