Word of mouth is most definitely the most powerful advertiser.
Back in the day, the R1 companies acknowledged that fansubs were a vital part of the process. People aren't just going to dish out $25 for some show based off some promotional art and videos. People watched fansubs, people talked about shows that they watched fansubbed, the anime community as a whole heard about the better shows, people bought said shows.
The problem is that people don't buy anymore.
However, getting rid of fansubs isn't going to get people to suddenly start buying shows based off promotional stuff. It'd be the final nail in the coffin, and they know that, which is why they aren't going around trying to shut down all of the fansubbing groups even when they have the right. The only way that the R1 industry is going to steady itself is to somehow get the anime community to watch anime in a way that profits them, which is looking difficult, thanks to the kind of mentality that you see in this thread, some twisted idea that watching fansubs alone helps the industry, or just general apathy. They probably don't have enough money to turn a project like a linear non-premium nation-wide anime channel that airs anime around the same time that they air in Japan (which is probably their only hope now) into a success anymore.