It's still up for the campaign managers to up their rates. After all, even if $ value of bitcoin goes down, would the quality of posts made by users go up if campaign managers raise their rates? They would still get the same level of advertising without any actual effect on their service, so why raise it when they really get what they want?
Increasing the rates probably means less qualitative posts and more spamming in general to earn as much money as possible

That's what it is. The quality per post wouldn't actually change in case campaign managers raise their rates. A campaign manager could actually check the posts of each user one by one, but that is such a tedious work to do on the manager's end.