I think people should be entitled to a free ride.
Important infrastructure can, paradoxically, be both free and expensive. We've committed, for instance, to spending a whole lot of money on maintaining all of our roads, but access to the street is still free for everyday pedestrians.
Not everyone can afford to pay to use the road. If you want to get a job, often you'll need to drive to and from work for a good while before you'll have enough cash built up to meaningfully contribute to the road through your taxes. Of course, wealthy people wind up paying more, but often times they wouldn't have ever become wealthy if roads hadn't originally been available to them for free. Important infrastructure can be free for individuals when our society makes a commitment to collectively paying for it.
MDN is an extremely important resource to beginner devs -- the exact people who would have trouble paying to support it. But supporting beginner devs is good for both the industry and the economy. Important public or open-source infrastructure such as MDN and Wikipedia should receive public grant money. We already do this with research grants for science, because there's a strong understanding that making research available for free makes society better. Informational infrastructure deserves the same consideration.