Jukkak is spot on. The relationship between flush and destruction is a case of mistaken causality.
A lot of techs who don't know much about transmissions (I am included in this group) will get handed work orders for poor shifting, or some other fairly minor issue, recommend flushing the trans because new fluid cleans and lubricates better, and often poor shifting is a sign of something getting stuck in the valve body. The customer approves the flush thinking it is a fix-all kind of thing, rather than a "Well let's start by getting your basic maintenance up to date" thing. Then because the issue was more serious it actually breaks a little while later, after it seemed good. They then assume, because all customers think this way, that the flush must have damaged something inside the trans. So they go tell their idiot brother in law, who tells his idiot co-workers, and pretty soon the whole world thinks that regular maintenance, especially if it was overdue, is going to break their car.
The truth of the matter is if you change your fluid and the trans breaks right away it was on borrowed time already because the dirt was all that was holding the guts together.