Sales in JUMP could've went down when they forced Double Arts to finish, they could've noticed that sales went down considerably when HXH went on all those hiatuses. I doubt they would automatically assume that their decline in sales is solely dependent on OP.
While in theory this might apply, we need to keep in mind that series are constantly coming and going in JUMP. This is something I didn't realize until I actually started buying it weekly years back.
When a new series started (the name of which I've even forgotten) I was so excited thinking I was on the verge of some new great new series. The thing went, MAYBE 20 chapters before being canned. If you follow what series have come and gone just in the past 2 years, you'd be shocked. In a long round-about way, I'm saying that those drops are natural. They don't hurt the serialization and they're not only expected, they're depended onto make room for more.
Now, you brought up the HxH breaks. I think it was a bragging point of the series that JUMP sales went up when it was around correct? I have no source for this, but granted from reactions I see EVERY day in the classroom and in the work place, the excitement surrounding the series is there and tension is definitely up when HxH is around.
In short, if sales drop, and stay stagnant they're going to know what was responsible. Will they know exactly how? No, and that's why I agree with Meh. Letter-writing and fan-supported movements are awesome. I totally support them. But in the midst of that, I'd never financially reward them for taking away a product I enjoyed.