So long as the docs themselves remain under CC-BY-SA though guess it's not really a problem. The premium features they currently offer seem reasonable, and if things ever get too out of hand the community could always just move to another site. It also doesn't sound like something I'd pay for though, so not sure how successful it'll be.
On the positive side, more funding to maintain MDN could turn out to be a good thing in the long term. I guess we'll have to see.