I agree that a huge problem with the Manabi line is that people don't put sales into context when considering profitability. There are plenty of shows that wouldn't seem successful based on volume sales but would be considered successes in terms of franchise promotion due to the merchandise they push or due to a boost in manga/light novel sales after the anime airs.
I also think that most people who argue that sales = quality had already reached a conclusion about the quality of something before they actually know the sales
. They just need an objective stance to argue with.
Edit: Oh, I haven't really seen an increase in people overvaluing sales data though. Maybe it's just the sites I visit. But overvaluing preorder/stalker data? That sounds even more unreliable and tumultuous!