[Thorrez]

>a single reasonable subscription

That subscription might be reasonable for a middle class person in the US, but way too expensive for the average person in a poorer country. Ads put everyone on the same footing.

And if you want to do research on a topic across many different websites, it'll happen that they won't all be in the same community, so you'll have to pay subscription fees to many different communities, sometimes just for a single page view.