There actually isn't REALLY such a thing as "swear words" in Japanese anyway, just really impolite forms of speech. So it's hard to say that they've actually "cut out swearing." But I get what you mean.
...But as you say, VIZ has done it for some time now, and plenty of companies before them, so it's not really something that's "changed."
I feel like there are less manga series to get excited about. It's sort of like we've mostly caught up and are releasing series that are big in Japan (even if we might be behind them in specific chapters/volumes), and now we're waiting with them for the next big breakout thing. Which might explain declining sales.
I mean, think about it...20 years ago, even 10 years ago there were still plenty of majorly popular series that hadn't been licensed. There aren't a lot of those now...and those that are there probably CAN'T be licensed or they already would have. But so, that means that 10 years ago, maybe a quarter or even half of new licensing announcements were ZOMG BIG DEAL kind of things. Now the new licenses are so new they haven't necessarily had time to build up audiences in Japan, which means more risk for the companies that license them and less interest from buyers, unless the series significantly breaks out.
I think that plays a part. I think it's true for the anime world as well.