I'm afraid that's not true at all. Airlines may well try to improve a delay, and unless they are highly confident they cannot (for example, a delay caused by legal requirement for crew rest with no other crew available) they will do so. Airlines want to be on time. If they are sure the delay will be long, they may contact you to tell you to arrive at the airport later, but that only applies if they contact you.

Apart from the AA incident mentioned above, I have experience (2 this year so far, more in previous years) where Easyjet reduced a flight delay after announcing it.

Airlines may reduce a delay after announcing it and most explicitly state that they may do so.