What if he just built a vanity project trans-fat factory and doesn’t want to loose face, so insists everyone must eat cheap trans-fats?
What if the autocrat decides to block access to a free-press and to jail anyone that dares to say ice cream makes you fat?
We can play what if scenarios all day but end result is always the same.
In an dictatorship you can’t remove a bad actor from power or even laugh at them.
In a democracy you can.
Imagine if Joe Biden went on TV tomorrow and said "we're taxing gasoline to USD100 per gallon to force private cars off the road. It's gonna be tough for a while, but you'll thank me in 2050 when the public transit is fully fleshed out and we hit a 1.5c climate goal." He'd be driven out of office in favour of someone who ran on a campaign of "let's roast the next generation so you can save a dime at the pump today." OTOH, Xi Jinping could probably get away with it.
Xi isn't enacting enlightened laws to protect the environment. Quite the opposite. He's opening more and more coal fired power stations. He doesn't care if the rivers overflow.
You're saying "but he could do good", he won't. Dictators never do.
I'm not sure most-- or even any-- people really know what's best for everyone. It gets even more complex when it turns into what's best for one group versus another, or what's best for today comes at a cost to be paid in 5-10-20 years.
It's easy to catapult yourself into office by promising people ice cream instead of broccoli. The leader you want has to be persuasive enough to convince voters they want broccoli. An autocrat can just mandate broccoli.