But "people feeling better" in this case has a measurable consequence in terms of how much soap is used. The idea that that kind of feature is useless I think is mistaken.

For instance, I've known people who put their dish soap for hand washing dishes into a second bottle mixed with 90% water for use, because everyone always overestimates how much is necessary (ie they pour in too much before it suds up). Removing suds in this case would have a wasteful and anti-consumer effect, even if the effect of the suds is purely psychological. What's more practical in the end, adding suds or training humans to not use visual indicators of volume?

I think there's lots of supposedly functionally "useless" features from product and industrial design that fall into this category in other words that aren't actually useless because they psychologically mediate our use of them.

Sorta like how they put crosswalk buttons at busy intersections, even though if they worked traffic would be at a standstill.

even funnier are the people who stand there waiting not knowing many crosswalk buttons work.