Most left leaning forums look negatively on profit motive, and reddit is largely very left wing. Whatever that means nowadays. It's reasonable to be wary of incentives, but sometimes that zeal is misplaced.

Having said that, I don't think any site can go mainstream and maintain a semblance of quality discourse. If nothing else, it will become botted and infiltrated by shills. But even without that, normies will ruin it much earlier than any sophisticated attacks are necessary.