It's far more important to trek to criticize and reflect modern society, which is a lot harder to do if your characters are living in a utopia.

I disagree... if anything, the opposite is true! Having "Federation utopia" makes it incredibly easy to critique modern society. Just introduce planets which have whatever element of modern society you want to comment on, and then draw a painfully obvious comparison to the perfection that is humanity in the 24th century, and boom, it's done! Heck, you could even make an entire alien race to critique an element of modern society like capitalism, not that anybody would do something that obvious :P

I feel like TOS and TNG lived on this a little too much, especially in early TNG seasons. It was what made DS9 so interesting when the writers flipped the script. Instead of spoon feeding you the critique of modern society in the form of planet-of-the week, they throw in stuff that makes you question whether the federation utopia approach is actually right, or if it's too naive.